System and method for inspecting a wafer

ABSTRACT

A method and a system for inspecting a wafer. The system comprises an optical inspection head, a wafer table, a wafer stack, a XY table and vibration isolators. The optical inspection head comprises a number of illuminators, image capture devices, objective lens and other optical components. The system and method enables capture of brightfield images, darkfield images, 3D profile images and review images. Captured images are converted into image signals and transmitted to a programmable controller for processing. Inspection is performed while the wafer is in motion. Captured images are compared with reference images for detecting defects on the wafer. An exemplary reference creation process for creating reference images and an exemplary image inspection process is also provided by the present invention. The reference image creation process is an automated process.

FIELD OF INVENTION

The present invention relates generally to a wafer inspection process.More specifically, the present invention relates to an automated systemand method for inspecting semiconductor components.

BACKGROUND

The ability to ensure a consistently high quality of manufacturedsemiconductor components, for example semiconductor wafers and dies, isincreasingly crucial in the semiconductor industry. Semiconductor waferfabrication techniques have been consistently improved for incorporatingan increasing number of features into a smaller surface area of thesemiconductor wafer. Accordingly, the photolithographic processes usedfor semiconductor wafer fabrication have become more sophisticated toallow the incorporation of increasing number of features into thesmaller surface area of the semiconductor wafer (i.e. for achievinghigher performance of the semiconductor wafers). Consequently, sizes ofpotential defects on semiconductor wafers are now typically in themicron to submicron range.

It is evident that manufacturers of semiconductor wafers have anincreasingly pressing need to improve semiconductor wafer qualitycontrol and inspection procedures to ensure a consistently high qualityof manufactured semiconductor wafers. Semiconductor wafers are typicallyinspected for detecting defects thereon, such as presence of surfaceparticulates, imperfections, undulations and other irregularities. Suchdefects could affect eventual performance of the semiconductor wafer.Therefore, it is critical to eliminate or extract defectivesemiconductor wafers during the manufacturing of semiconductor wafers.

There have been advances in semiconductor inspection systems andprocesses. For example, higher resolution imaging systems, fastercomputers, and enhanced precision mechanical handling systems have beencommissioned. In addition, semiconductor wafer inspection systems,methods and techniques have historically utilized at least one ofbrightfield illumination, darkfield illumination and spatial filteringtechniques.

With brightfield imaging, small particles on the semiconductor waferscatter light away from a collecting aperture of an image capturedevice, thereby resulting in a reduction of returned energy to the imagecapture device. When the particle is small in comparison with theoptical point spread function of a lens or digitalizing pixel,brightfield energy from the immediate areas surrounding the particlegenerally contribute a large amount of energy relative to the particle,thereby making the particle difficult to detect. In addition, the verysmall reduction in energy due to the small particle size is often maskedby reflectivity variations from the immediate areas around the particlethereby resulting in increased occurrences of false defect detection. Toovercome the above phenomena, semiconductor inspection systems have beenequipped with high-end cameras with larger resolutions, which captureimages of smaller surface areas of the semiconductor wafer. Brightfieldimages generally have a better pixel contrast and this is advantageousfor estimating size of defects and when inspecting dark defects.

Darkfield imaging and its advantages are generally well-known in theart. Darkfield imaging has been employed with several existingsemiconductor wafer inspection systems. Darkfield imaging typicallydepends on the angle at which light rays are incident on the object tobe inspected. At a low angle to a horizontal plane of the object to beinspected (for example 3 to 30 degrees), darkfield imaging typicallyproduces a dark image except at locations where defects, such as surfaceparticulates, imperfections and other irregularities exist. A particularuse of darkfield imaging is to light up defects which sizes are smallerthan the resolving power of lens used to produce brightfield images. Ata higher angle to the horizontal plane (for example 30 to 85 degrees),darkfield imaging typically produces better contrast images as comparedto brightfield images. A particular use of such high angle darkfieldimaging enhances contrast of surface irregularities on a mirror finishor transparent object. In addition, high angle darkfield imagingenhances imaging of tilted objects.

Light reflectivity of the semiconductor wafer typically has asignificant effect on quality of images obtained with each ofbrightfield and darkfield imaging. Both micro and macro structurespresent on the semiconductor wafer affect the light reflectivity of thesemiconductor wafers. Generally, amount of light reflected by thesemiconductor wafer is a function of the direction or angle of incidentlight, the viewing direction and the light reflectivity of the surfaceof the semiconductor wafer. The light reflectivity is in turn dependenton wavelength of the incident light and material composition of thesemiconductor wafer.

It is generally difficult to control the light reflectivity ofsemiconductor wafers presented for inspection. This is because thesemiconductor wafer may consist of several layers of material. Eachlayer of material may transmit different wavelengths of lightdifferently, for example at different speeds. In addition, layers mayhave different light permeabilities, or even reflectivity. Accordingly,it will be apparent to a person skilled in the art that the use of lightor illumination of a single wavelength or a narrow band of wavelengthstypically adversely affects quality of captured images. Need forfrequent modification of the single wavelength or narrow band ofwavelengths requires use of multiple spatial filters or wavelengthtuners, which is generally inconvenient. To alleviate such problems, itis important to use a broadband illumination (i.e. illumination of abroad or wide range of wavelengths), for example broadband illuminationof a range of wavelengths between 300 nm and 1000 nm.

Broadband illumination is important for achieving high quality images aswell as for inspecting semiconductor wafers with a wide range of surfacereflectivities. In addition, defect detection capabilities of waferinspection systems will generally be enhanced by use of multipleillumination angles or contrasts, for example use of both brightfieldand darkfield illuminations. Existing wafer systems in the markettypically do not utilize illuminations of multiple angles and with afull broadband wavelength.

Currently available wafer inspection systems or equipments typically useone of the following methods for achieving multiple responses duringwafer inspection:

(1) Multiple Image Capture Devices with Multiple Illuminations (MICD)

The MICD uses a plurality of image capture devices and a plurality ofilluminations. The MICD is based on the principle of segmenting thewavelength spectrum into narrow bands, and allocating each segmentedwavelength spectrum to individual illuminations. During design ofsystems employing the MICD method, each image capture device is pairedwith a corresponding illumination (i.e. illumination source), togetherwith corresponding optical accessories such as a spatial filter or aspecially coated beam splitter. For example, wavelength of thebrightfield illumination is limited between 400 to 600 nm using amercury arc lamp and a spatial filter and the wavelength of thedarkfield illumination is limited between 650 to 700 nm using lasers.The MICD method experiences disadvantages, for example inferior imagequality and a relative inflexibility in system design or configuration.The inferior image quality is generally due to varying surfacereflectivities of inspected semiconductor wafers, combined with the useof illuminations with narrow wavelengths for inspecting thesemiconductor wafers. Design inflexibility of the system occurs becausethe modification of the wavelength of a single illumination used withthe system typically requires reconfiguration of the entire opticalsetup of the system. In addition, the MICD method typically does noteasily enable capture of illuminations of varying wavelengths by asingle image capture device without compromising the quality of capturedimages or the speed of capturing images.

(2) Single Image Capture Device with Multiple Illuminations (SICD)

The SICD method uses a single image capture device for capturingmultiple illuminations, each of the multiple illuminations being eitherof segmented wavelengths (i.e. narrow band of wavelengths) or ofbroadband wavelengths. However, it is not possible to obtain multipleillumination responses simultaneously while the semiconductor wafer isin motion. In other words, the SICD method only allows one illuminationresponse when the semiconductor wafer is in motion. To achieve multipleillumination responses, the SICD method requires capture of images whilethe semiconductor wafer is stationary, which affects throughput of thewafer inspection system.

Semiconductor wafer inspection systems employing simultaneous,independent, on-the-fly image capture using broadband brightfieldillumination and darkfield illumination, or in general multipleilluminations, and using multiple image capture devices are notpresently available due to a relative lack of understanding as to actualimplementation and operating advantages thereof.

As described above, existing semiconductor wafer inspection systemstypically employ either MICD or SICD. Equipments employing MICD do notuse broadband illuminations and typically suffer from inferior imagequality and inflexibility in system setup or configuration. On the otherhand, semiconductor wafer inspection systems using SICD experiencediminished system throughput and are incapable of obtaining on-the-flysimultaneous multiple illumination responses.

An exemplary existing semiconductor wafer optical inspection system thatutilizes both brightfield illumination and darkfield illuminator isdisclosed in U.S. Pat. No. 5,822,055 (KLA1). An embodiment of theoptical inspection system disclosed in KLA1 utilizes MICD as describedabove. The optical inspection system disclosed in KLA1 uses multiplecameras to capture separate brightfield and darkfield images ofsemiconductor wafers. Captured brightfield and darkfield images are thenprocessed separately or together for detecting defects on thesemiconductor wafers. In addition, the optical inspection system of KLA1captures brightfield and darkfield images simultaneously using separatesources of brightfield and darkfield illumination. The opticalinspection system of KLA1 achieves simultaneous image capture (i.e.capture of brightfield and darkfield images) by using illuminationemitters emitting illuminations of segmented wavelength spectrums andspatial filters. With the optical inspection system of KLA1, one of thecameras is configured to capture darkfield images with corresponding useof a narrow band laser illumination and spatial filter. Another camerais configured to capture brightfield images with corresponding use ofbrightfield illumination and a beam splitter having a special coating.Disadvantages of the optical inspection system disclosed by KLA1 includeunsuitability thereof for imaging semiconductor wafers comprising alarge variation of surface reflectivities. This is due to use ofilluminations of segmented wavelength spectrums. The cameras are eachfor capturing illumination of a predetermined wavelength spectrum. Thereis little flexibility for each camera to capture illuminations ofmultiple different wavelength spectrums for enhancing captured images ofcertain wafer types. For example, wafers comprising a carbon-coatedlayer on their first surface exhibit poor reflection characteristics atcertain illumination angles, for example with brightfield illumination.Accordingly, a combination of brightfield illumination and high angledarkfield illumination is required for viewing certain defects on suchwafers. The optical inspection system of KLA1 utilizes a plurality ofillumination emitters or sources and filters. The optical inspectionsystem of KLA1 performs multiple inspection passes (i.e. multiple scans)to thereby enable the capture of both brightfield and darkfield images.Consequently, the throughput of the optical inspection system isadversely affected.

Additional exemplary existing optical inspection systems utilizing bothbrightfield and darkfield imaging are disclosed in U.S. Pat. No.6,826,298 (AUGTECH1) and U.S. Pat. No. 6,937,753 (AUGTECH2). The opticalinspection systems of AUGTECH1 and AUGTECH2 utilize a plurality oflasers for performing low angle darkfield imaging, and a fiber opticring light for performing high angle darkfield imaging. In addition, theoptical inspection systems of AUGTECH1 and AUGTECH2 each utilizes asingle camera sensor and the SICD method as explained earlier.Accordingly, inspection of semiconductor wafers by the opticalinspection systems of AUGTECH1 and AUGTECH2 is performed either bybrightfield imaging or by darkfield imaging or via a combination of bothbrightfield imaging and darkfield imaging wherein each of thebrightfield imaging and darkfield imaging is performed when the other iscompleted. The inspection system of AUGTECH1 and AUGTECH2 is not capableof simultaneous, on-the-fly, and independent brightfield and darkfieldimaging. Accordingly, multiple passes of each semiconductor wafer arerequired for completing inspection thereof. This results in loweredmanufacturing throughput and an increased utilization of resources.

In addition, several existing optical inspection systems utilize agolden image or a reference image for comparison with newly acquiredimages of semiconductor wafers. Derivation of the reference imagetypically involves capturing several images of known or manuallyselected “good” semiconductor wafers, and then applying a statisticalformula or technique to thereby derive the reference image. Adisadvantage with the above derivation technique is inaccuracies orinconsistencies associated with manual selection of the “good”semiconductor wafers. Optical inspection systems using such referenceimages can suffer from false rejects of semiconductor wafers due toinaccurate or inconsistent reference images. With increasingly complexcircuit geometry of semiconductor wafers, the reliance on manualselection of “good” semiconductor wafers for deriving reference imagesis becoming increasingly incompatible, particularly with the increasingquality standards set by the semiconductor inspection industry.

Deriving a golden reference image involves many statistical techniquesand calculations. Most of existing statistical techniques are verygeneral and have their own merits. Currently available opticalinspection systems or equipment typically use either average or meantogether with standard deviation when deriving a golden reference pixel.Use of mean with standard deviation for deriving golden reference pixelscan be useful with known good pixels; otherwise any defect or noisepixel would interfere and affect final average or mean value of thereference pixel. Another statistical technique utilizes median forreducing interference due to noise pixel. However, it is not possible,or at least difficult, to substantially eliminate the effect of noise.Existing optical inspection systems or equipment try to reduce theeffect of noise by applying varying statistical techniques. However, auser friendly or simple method for reducing or eliminating the effect ofnoise (i.e. error) has yet to be devised. Such a method will help toeliminate noise pixels, which would affect the final reference pixelvalue.

U.S. Pat. No. 6,324,298 (AUGTECH3) discloses a training method forcreating a golden reference or reference image for use in semiconductorwafer inspection. The method disclosed in AUGTECH3 requires “Known GoodQuality” or “Defect Free” wafers. Selection of such “Known Good Quality”wafers is manually or user performed. Statistical formulas or techniquesare then applied for deriving the reference image. As such, accurate andconsistent selection of “Known Good Quality” wafers is crucial formaintaining a high quality of semiconductor inspection. The method ofAUGTECH3 uses mean and standard deviation to calculate individual pixelsof the reference image. Accordingly, presence of any defective pixelwill lead to inaccurate derivation of reference pixel. The defectivepixel can occur due to foreign matter or other defects. Such foreignmatter or defects could adversely affect the statistical calculation andlead to inaccurate derivation of reference pixel. It will be apparent toa person skilled in the art that the method of AUGTECH3 is open toinaccuracies, inconsistencies and errors in inspection of thesemiconductor wafers.

In addition, optical inspection system disclosed in AUGTECH3 uses aflash or strobe lamp for illuminating the semiconductor wafers. It willbe appreciated by a person skilled in the art that inconsistenciesbetween different flashes or strobes may occur due to numerous factorsincluding, but not limited to, temperature differentials, electronicinconsistencies and differential flash or strobe intensities. Suchdifferentials and inconsistencies are inherent even with “good”semiconductor wafers. Presence of such differentials would affect thequality of derived golden reference images if the system does not takesuch differentials into consideration. In addition, illuminationintensity and uniformity varies across the surface of the semiconductorwafer due to factors including, but not limited to, differing planarityof the wafer, mounting and light reflectivities at different positionson the surface of the semiconductor wafer. Without taking into accountthe above-mentioned differentials and factors, any reference imagesderived in the above-described manner may be unreliable and inaccuratewhen used for comparison with captured images at different positions onthe surface of the semiconductor wafers.

Variations in product specifications, for example semiconductor wafersize, complexity, surface reflectivity, are common in the semiconductorindustry. Accordingly, semiconductor wafer inspection systems andmethods need to be capable of inspecting semiconductor wafers ofdifferent specifications. However, existing semiconductor waferinspection systems and methods are generally incapable of satisfactorilyinspecting semiconductor wafers of a wide range of differentspecifications, especially given the increasing quality standards set bythe semiconductor industry.

For example, a typical existing semiconductor wafer inspection systemuses a conventional optical assembly comprising components, for examplecameras, illuminators, filters, polarizers, mirrors and lens, which havefixed spatial positions. Introduction or removal of components of theoptical assembly generally requires rearrangement and redesign of theentire optical assembly. Accordingly, such semiconductor waferinspection systems have inflexible designs or configurations, andrequire a relatively long lead-time for modification thereof. Inaddition, distance between objective lens of the convention opticalassembly and semiconductor wafer presented for inspection is typicallytoo short to allow ease of introduction of fiber optics illuminationwith differing angles for facilitating darkfield imaging.

There are numerous other existing semiconductor wafer inspection systemsand methods. However, because of current lack of technical expertise andoperational know-how, existing semiconductor wafer inspection systemscannot employ simultaneous brightfield and darkfield imaging for aninspection while the wafer is in motion, while still having design andconfigurationally flexible. There is also a need for semiconductor waferinspection systems and methods for enabling resource-efficient,flexible, accurate and fast inspection of semiconductor wafers. This isespecially given the increasing complexity of electrical circuitry ofsemiconductor wafers and the increasing quality standards of thesemiconductor industry.

SUMMARY

There is currently a lack of semiconductor wafer inspection systems andmethods capable of employing both brightfield and darkfield imaging forinspecting a semiconductor wafer while the semiconductor wafer is inmotion, in addition to facilitating system configurational and designflexibility. In addition, there is need for a semiconductor waferinspection system wherein components thereof, for example illuminators,camera, objective lens, filters and mirrors, have flexible andadjustable spatial interconfigurations. Given the increasing complexityof electrical circuitry of semiconductor wafers, and the increasingquality standards set by the semiconductor industry, accuracy andconsistency of semiconductor wafer inspection is increasingly critical.Derivation of golden references or reference images for comparison withcaptured images of semiconductor wafers currently require manualselection of “good” semiconductor wafers. Such a manual selection canresult in inaccuracies and inconsistencies in the derived referenceimages, and therefore consequent inspection of semiconductor wafers.Accordingly, there is a need for improved training methods or processesfor deriving reference images to which subsequent captured images ofsemiconductor wafers can be compared. The present invention seeks toaddress at least one of the above-described issues.

The present invention provides an inspection system and method forinspecting semiconductor components, including, but not limited tosemiconductor wafers, dies, LED chips and solar wafers. The inspectionsystem is designed for performing 2-Dimensional (2D) and 3-Dimensional(3D) wafer inspection. The inspection system is further designed forperforming defect review.

The 2D wafer inspection is facilitated by a 2D optical module, whichcomprises at least two image capture devices. The 2D wafer inspectionutilizes at least two different contrast illuminations for capturingimages of corresponding contrast illuminations. The 2D wafer inspectionis performed while the semiconductor wafer is in motion, and can becompleted with one pass of the semiconductor wafer. The 3D waferinspection is facilitated by a 3D optical module, which comprises atleast one image capture device and at least one thin line illuminator orthin line illumination emitter. Thin line illumination supplied by thethin line illuminator, which is either laser or broadband illuminationsource or both, is directed at the semiconductor wafer while thesemiconductor wafer is in motion for capturing 3D images of thesemiconductor wafer. Defect review performed by the inspection system isfacilitated by a defect review optical module.

In accordance with a first aspect of the present invention, there isdisclosed a method comprising capturing a first image of a wafer under afirst contrast illumination and capturing a second image of the waferunder a second contrast illumination, each of the first illumination andthe second illumination having a broadband wavelength, the firstcontrast illumination and the second contrast illumination for enablingdetection of at least one defect site in the first and second images.The wafer is spatially displaced by a predetermined distance between thecapture of the first image and the capture of the second image. Themethod further comprises correlating the first and second images andcomparing the defect sites in the first image with the defect sites inthe second image for producing an identification of a defect.

In accordance with a second aspect of the present invention, there isdisclosed a method comprising providing a first image of a wafer, thefirst image having one or more defect sites and providing a second imageof the wafer, the second image having one or more defect sites and thewafer being spatially displaced between the providing of the first andsecond image. The method further comprises correlating the spatialdisplacement of the wafer with the first and second image and comparingthe defect sites of the first image with the defect sites of the secondimage for producing an identification of a defect.

In accordance with a third aspect of the present invention, there isdisclosed a method comprising capturing a first image of a wafer under afirst contrast illumination and capturing a second image of the waferunder a second contrast illumination, the first contrast illuminationand the second contrast illumination for enabling detection of at leastone defect site in the first and second images. The wafer is spatiallydisplaced by a predetermined distance between the capture of the firstimage and the capture of the second image. The method further comprisescorrelating the first and second images and comparing the defect sitesin the first image with the defect sites in the second image forproducing an identification of a defect.

In accordance with a fourth aspect of the present invention, there isdisclosed a system comprising a first image capture module for capturinga first image of a wafer and a second image capture module for capturinga second image of the wafer, the wafer being spatially displaced by apredetermined distance between the capture of the first image and thecapture of the second image. The system further comprises a defect sitecomparison module coupled to the first and second image capture module,the defect site comparison module for correlating the first and secondimage with the spatial displacement of the wafer, comparing a defectsite detected in the first image with another defect site detected inthe second image and producing an identification of a defect therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are described hereinafterwith reference to the following drawings, in which:

FIG. 1 shows a partial plan view of an exemplary system for inspectingwafers according to an exemplary embodiment of the present invention;

FIG. 2 shows a partial isometric view of the system of FIG. 1;

FIG. 3 shows an exploded partial isometric view of an optical inspectionhead of the system of FIG. 1 according to view “A” highlighted in FIG.2;

FIG. 4 shows an exploded partial isometric view of a robotic wafer tableof the system of FIG. 1 according to view “B” highlighted in FIG. 2;

FIG. 5 shows an exploded partial isometric view of a robotic waferhandler of the system of FIG. 1 according to view “C” highlighted inFIG. 2;

FIG. 6 shows an exploded partial isometric view of a wafer stack moduleof the system of FIG. 1 according to view “D” highlighted in FIG. 2;

FIG. 7 shows a partial isometric view of the optical inspection head ofthe system of FIG. 1;

FIG. 8 shows a partial front view of the optical inspection head of thesystem of FIG. 1;

FIG. 9 shows optical ray paths of illumination between a brightfieldilluminator, a low angle darkfield illuminator, a high angle darkfieldilluminator, a first image capture device and a second image capturedevice of the system of FIG. 1;

FIG. 10 is a flowchart of an exemplary first ray path followed by thebrightfield illumination supplied by the brightfield illuminator of FIG.9;

FIG. 11 is a flowchart of an exemplary second ray path followed by thedarkfield high angle illumination supplied by the high angle darkfieldilluminator of FIG. 9;

FIG. 12 is a flowchart of an exemplary third ray path followed by thedarkfield low angle illumination supplied by the low angle darkfieldilluminator of FIG. 9;

FIG. 13 shows optical ray path of illumination between a thin lineilluminator and a 3D image capture device or camera of the system ofFIG. 1;

FIG. 14 shows optical ray path of illumination between a reviewbrightfield illuminator, a review darkfield illuminator and a reviewimage capture device of the system of FIG. 1;

FIG. 15 is a flowchart of an exemplary fourth ray path followed bybrightfield illumination between the review brightfield illuminator andthe review image capture device of FIG. 14;

FIG. 16 is a flowchart of an exemplary fifth ray path followed bydarkfield illumination between the review darkfield illuminator and thereview image capture device of FIG. 14;

FIG. 17 is a method flow diagram of an exemplary method for inspectingwafers provided by the present invention;

FIG. 18 is a process flowchart of an exemplary reference image creationprocess for creating reference images used for comparing with imagescaptured during performance of the method of FIG. 17;

FIG. 19 is a process flow diagram of an exemplary two-dimensioned waferscanning process with timing offset performed in a method step of themethod of FIG. 17;

FIG. 20 shows a table of illumination configurations selectable by anillumination configurator of the system of FIG. 1;

FIG. 21 shows a timing chart for capturing of a first image by the firstimage capture device and capturing of a second image by the second imagecapture device performed in the two-dimensioned wafer scanning processof FIG. 19;

FIG. 22a shows the first image captured by the first image capturedevice of FIG. 1;

FIG. 22b shows the second image captured by the second image capturedevice of FIG. 1;

FIG. 22c shows a combined first image of FIG. 22a and second image ofFIG. 22b for demonstrating image offset due to the capture of the firstimage and the second image when the wafer is moving;

FIG. 23 is a process flow diagram of an exemplary two dimensional imageprocessing process performed in a method step of the method of FIG. 17;

FIG. 24 is a process flow diagram of a first exemplary three dimensionalwafer scanning process performed in a method step of the method of FIG.17;

FIG. 25 shows an exemplary optical ray path of illumination between athin line illuminator a 3D image capture device or camera of the systemof FIG. 1;

FIG. 26 is a process flow diagram of a second exemplary threedimensional wafer scanning process performed in a method step of themethod of FIG. 17; and

FIG. 27 is a process flow diagram of an exemplary review processperformed in a method step of the method of FIG. 17.

DETAILED DESCRIPTION

The inspection of semiconductor components, for example semiconductorwafers and dies, is an increasingly critical step in the manufacture orfabrication of semiconductor components. Increasing complexity ofcircuitry of semiconductor wafers, coupled with increasing qualitystandards for semiconductor wafers, has led to an increasing need forimproved semiconductor wafer inspection systems and methods.

There is currently a lack of semiconductor wafer inspection systems andmethods capable of employing both brightfield and darkfield imagingsimultaneously for performing on-the-fly inspection of semiconductorwafers, while providing configurational or design flexibility. Inaddition, there is need for a semiconductor wafer inspection systemwherein components thereof, for example illuminators, cameras or imagecapture devices, objective lens, filters and mirrors, have flexible andadjustable spatial interconfigurations. Given the increasing complexityof electrical circuitry of semiconductor wafers, and the increasingquality standards set by the semiconductor industry, accuracy andconsistency of semiconductor wafer inspection is increasingly critical.Derivation of golden references or reference images for comparison withcaptured images of semiconductor wafers currently require manualselection of “good” semiconductor wafers. Such a manual selection canresult in inaccuracies and inconsistencies in the derived referenceimages, and consequently in the inspection of semiconductor wafers.Accordingly, there is a need for improved training methods or processesfor deriving reference images to which subsequently captured images ofsemiconductor wafers can be compared.

The present invention provides exemplary systems and methods forinspecting semiconductor components for addressing at least one of theabove-identified issues.

For purposes of brevity and clarity, the description of the presentinvention is limited hereinafter to systems and methods for inspectingsemiconductor wafers. It will however be understood by a person skilledin the art that this does not preclude the present invention from otherapplications where fundamental principles prevalent among the variousembodiments of the present invention such as operational, functional orperformance characteristics are required. For example, the systems andmethods provided by the present invention can be used for inspectingother semiconductor components including, but not limited to,semiconductor dies, LED chips and solar wafers.

An exemplary system 10 for inspecting semiconductor wafers 12 as shownin FIG. 1 and FIG. 2 is provided according to a first embodiment of thepresent invention. The system 10 can also be used for inspecting othersemiconductor devices or components as required. Preferably, the system10 comprises an optical inspection head 14 (as shown in FIG. 3), a wafertransportation table or wafer chuck 16 (as shown in FIG. 4), a roboticwafer handler 18 (as shown in FIG. 5), a wafer stack module 20 (as shownin FIG. 6) or film frame cassette holder, an X-Y displacement table 22,and at least one set of quad vibration isolators 24 (as shown in FIG. 1and FIG. 2).

The optical inspection head 14 as shown in FIG. 7 and FIG. 8 comprises anumber of illuminators and image capture devices. Preferably, theoptical inspection head 14 comprises a brightfield illuminator 26, a lowangle darkfield illuminator 28 and a high angle darkfield illuminator30. It will be understood by a person skilled in the art that additionaldarkfield illuminators may be incorporated into the system 10 asrequired. It will be further understood by a person skilled in the artthat the low angle darkfield illuminator 28 and the high angle darkfieldilluminator 30 may be integrated as a single darkfield illuminator,which may be flexibly positioned as required.

The brightfield illuminator 26, also known as a brightfield illuminationsource or a brightfield illumination emitter, supplies or emitsbrightfield illumination or light. The brightfield illuminator 26 is forexample, a flash lamp or a white light emitting diode. Preferably, thebrightfield illuminator 26 supplies broadband brightfield illuminationcomprising wavelengths of substantially between and including 300 nm and1000 nm. It will however be understood by a person skilled in the artthat the brightfield illumination may be of alternative wavelengths andoptical properties.

The brightfield illuminator 26 preferably comprises a first opticalfiber (not shown) through which the brightfield illumination travelsbefore being emitted from the brightfield illuminator 26. Preferably,the first optical fiber acts as a waveguide for guiding direction oftravel of brightfield illumination. Further preferably, the firstoptical fiber facilitates directing of the brightfield illuminationemitted from the brightfield illuminator 26.

The low angle darkfield illuminator 28 and the high angle darkfieldilluminator 30 are also known as darkfield illumination sources ordarkfield illumination emitters, and supplies or emits darkfieldillumination. Darkfield illuminators are carefully aligned illuminationor light sources which enable minimization of the quantity of directlytransmitted (or un-scattered) light entering their corresponding imagecapture devices. Generally, image capture devices for capturingdarkfield images receives only illumination or light that has beenscattered by a sample or object. Darkfield images generally haveenhanced image contrast as compared to brightfield images. Brightfieldillumination and darkfield illumination are examples of contrastilluminations.

The low angle darkfield illuminator 28 and the high angle darkfieldilluminator 30 are for example flash lamps or white light emittingdiodes. Preferably, the darkfield illumination supplied by each of thelow angle darkfield illuminator 28 and the high angle darkfieldilluminator 30 share similar optical properties as the brightfieldillumination. More specifically, the darkfield illumination supplied byeach of the low angle darkfield illuminator 28 and the high angledarkfield illuminator 30 is preferably a broadband darkfieldillumination (also referred to as a darkfield broadband illumination)comprising a wavelength of substantially between and including 300 nm to1000 nm. This is to say that the brightfield illumination and thedarkfield illumination are broadband illuminations. Alternatively, thelow angle darkfield illuminator 28 and the high angle darkfieldilluminator 30 supply or emit darkfield illumination of differentwavelengths or other optical properties.

The low angle darkfield illuminator 28 is positioned at a lower angle,as compared to the high angle darkfield illuminator 30, to a horizontalplane of the semiconductor wafer 12 placed on the wafer table 16 (or toa horizontal plane of the wafer table 16). For example, the low angledarkfield illuminator 28 is preferably positioned at an angle of betweenthree and thirty degrees to the horizontal plane of the semiconductorwafer 12 placed on the wafer table 16. In addition, the high angledarkfield illuminator 30 is preferably positioned at an angle of betweenthirty and eighty-five degrees to a horizontal plane of thesemiconductor wafer 12 placed on the wafer table 16. The above-statedangles are preferably alterable as required by adjusting the position ofeach of the low angle darkfield illuminator 28 and the high angledarkfield illuminator 30.

Each of the low angle darkfield illuminator 28 and the high angledarkfield illuminator 30 preferably comprises a second and third opticalfiber (not shown) through which the darkfield illumination travelsbefore being emitted therefrom. Both the second and third optical fibersact as a waveguide for guiding direction of travel of the darkfieldillumination through each of the low angle darkfield illuminator 28 andthe high angle darkfield illuminator 30. In addition, the second opticalfiber facilitates directing of the darkfield illumination emitted fromthe low angle darkfield illuminator 28 and the third optical fiberfacilitates directing of the darkfield illumination emitted from thehigh angle darkfield illuminator 30. Illumination supplied by each ofthe brightfield illuminator 26, the low angle darkfield illuminator 28and the high angle darkfield illuminator 30 can be controlled, and canbe either continuously supplied or pulsed.

The wavelength spectrums of both the brightfield illumination anddarkfield illuminations preferably enhance accuracy of inspection anddefect detection of the wafers 12. Broadband illumination preferablyenables identification of a wide range of wafer defect types withvarying surface reflectivities. In addition, the broadband wavelengthsof both the brightfield illumination and the darkfield illuminationsenable the inspection of the wafer 12 to be performed independent ofreflective characteristics of the wafer 12. This means that thedetection of defects on the wafer 12 will preferably not be undesirablyinfluenced due to different sensitivities or reflectiveness orpolarization of the wafer 12 to different illumination wavelengths.

Preferably, intensities of the brightfield illumination and thedarkfield illumination supplied by the brightfield illuminator 26 andthe darkfield illuminators 28, 30 respectively can be selected andvaried as required depending on wafer 12 characteristics, for examplematerial and surface coating of the wafer 12. In addition, theintensities of each of the brightfield illumination and darkfieldilluminations can be selected and varied as required for enhancingquality of images captured of the wafer 12, and for enhancing quality oraccuracy of inspection of the wafer 12.

As shown in FIG. 7 to FIG. 9, the system 10 further comprises a firstimage capture device 32 (i.e. a first camera) and a second image capturedevice 34 (i.e. a second camera). Each of the first image capture device32 and the second image capture device 34 is capable of receivingbrightfield illumination supplied by the brightfield illuminator 26 andthe darkfield illuminations supplied by each of the low angle darkfieldilluminator 28 and high angle darkfield illuminator 30. Brightfield anddarkfield illuminations received by or entering the first image capturedevice 32 is preferably focused onto a first image capture plane (notshown) for capture of corresponding images. Brightfield and darkfieldilluminations received by or entering the second image capture device 34is preferably focused on a second image capture plane (not shown) forcapture of corresponding images.

The first image capture device 32 and the second image capture device 34capture either monochromatic or color images. The first image capturedevice 32 and the second image capture device 34 are alternatively knownas image capture modules or image sensors. Preferably, the ability tocapture, using single or three chip color sensor, color images of thewafer 12 enhances at least one of accuracy and speed of defectdetection. For example, the ability to capture color images of the wafer12 preferably helps to reduce false detection of defects on the wafer12, and correspondingly false rejection thereof.

The optical inspection head 14 further comprises a first tube lens 36for use with the first image capture device 32. In addition, the opticalinspection head 14 further comprises a second tube lens 38 for use withthe second image capture device 34. Each of the first tube lens 36 andthe second tube lens 38 preferably share common optical characteristicsand functions. Accordingly, the tube lenses 36 and 38 have been labeledthe first tube lens 36 and the second tube lens 38 solely for purposesof clarity. The objective lens mechanism comprises a number of objectivelenses 40, for example four objective lenses 40. The objective lenses 40are collectively mounted on a rotatable mount 42 (as shown in FIG. 3),which is rotatable for positioning each of the number of objective lens40 above an inspection position (not shown) or wafer 12 positioned forinspection. The objective lenses 40 and the rotatable mount 42 can becollectively referred to as an objective lens mechanism or objectivelens assembly.

Each of the number objective lenses 40 is used to achieve differentmagnification and is parfocal. Each of the number of objective lens 40is preferably of a different predetermined magnification factor, forexample five times, ten times, twenty times, and fifty times.Preferably, each of the number of objective lenses 40 has a correctedaberration in infinity. It will however be understood by a personskilled in the art that each of the number of objective lenses can bechanged or redesigned to achieve different magnification andperformance.

Each of the low angle darkfield illuminator 28 and the high angledarkfield illuminator 30 preferably comprises focusing means ormechanisms for directing or focusing the darkfield illuminationtherefrom towards the wafer 12 positioned at the inspection position.The angle between the low angle darkfield illuminator 28 and thehorizontal plane of the wafer 12 and the angle between the high angledarkfield illuminator 30 and the horizontal plane of the wafer 12 arepreferably determined and adjustable for enhancing accuracy of defectdetection. Preferably, each of the low angle darkfield illuminator 28and the high angle darkfield illuminator 30 has a fixed spatial positionwith reference to the inspection position. Alternatively, the positionof each of the low angle darkfield illuminator 28 and the high angledarkfield illuminator 30 is variable with reference to the inspectionposition during normal operation of the system 10.

As described above, both the brightfield illumination and the darkfieldilluminations are focused at the inspection position. The brightfieldillumination and the darkfield illuminations focused at the inspectionposition illuminates the wafer 12, or the portion thereof, positioned atthe inspection position.

As shown in FIG. 6, the system 10 comprises a wafer stack 20 or filmframe cassette holder. The wafer stack 20 preferably comprises slots tohold multiple wafers 12. Each of the multiple wafers 12 are sequentiallyloaded or transferred onto the wafer table 16 (as shown in FIG. 4) orwafer chuck by the robotic wafer handler 18 (as shown in FIG. 5).Preferably, a suction or vacuum is applied to the wafer table 16 forsecuring the wafer 12 thereonto. The wafer table 16 preferably comprisesa predetermined number of small holes or apertures through which vacuumis applied to enables a reliable and flat position of a flex frame tapeand a frame (both not shown) onto the wafer table 16. The wafer table 16is also preferably designed to handle wafer sizes of a range between andincluding six and twelve inches in diameter.

The wafer table 16 is coupled to the XY-displacement table 22 (as shownin FIG. 1 and FIG. 2), which enables the displacement of the wafer table16 in an X- and a Y-direction. Displacement of the wafer table 16correspondingly displaces the wafer 12 placed thereon. Preferably, thedisplacement of the wafer table 16, and hence displacement of the wafer12 placed thereon, is controlled for controlling the positioning of thewafer 12 at the inspection position. The XY-displacement table 22 isalternatively known as an air-gap linear positioner. The XY-displacementtable 22 or air-gap linear positioner facilitates high precisiondisplacement of the wafer table 16 in the X- and Y-directions withminimal effect of vibration transmitted from the rest of the system 10to the wafer table 16 and ensures smooth and accurate positioning of thewafer 12, or a portion thereof, at the inspection position. Assembly ofthe XY-displacement table 22 and wafer table 16 is mounted on thedampeners or vibration isolators 24 (as shown in FIG. 2) to absorbshocks or vibrations, and to ensure flatness of the assembly and othermodules or accessories mounted thereon. It will be appreciated by aperson skilled in the art that alternative mechanisms or devices may becoupled to or used with the wafer table 16 controlling the displacementthereof, and for facilitating high precision fine positioning of thewafer 12 at the inspection position.

The inspection of the wafer 12 for detecting possible defects thereon isperformed while the wafer 12 is in motion. This is to say, the captureof images, for example brightfield images and darkfield images, of thewafer 12 preferably occurs as the wafer 12 is being displaced across theinspection position. Alternatively, the wafer 12 may be held stationaryat the inspection position and positioned as required to capturehigh-resolution images. Control of displacement or movement of the wafer12 during inspection can be software controlled.

As previously mentioned, the system 10 further comprises the first tubelens 36 and the second tube lens 38. Preferably, the tube lens 36 ispositioned between the objective lenses 40 and the first image capturedevice 32. Illumination passes through the first tube lens 36 beforeentering the first image capture device 32. Further preferably, thesecond tube lens 38 is positioned between the objective lenses 40 andthe second image capture device 34. Illumination passes through thesecond tube lens 38 and deflected by a mirror or prism 47 beforeentering the second image capture device 34.

Each of the number of objective lenses 40 has a corrected aberration ininfinity. Accordingly, after passing through the objective lens 40,illumination or light is collimated. This is to say, illuminationtraveling between the objective lens 40 and each of the first tube lens36 and second tube lens 38 is collimated. The collimation ofillumination between the objective lens 40 and each of the first tubelens 36 and the second tube lens 38 enhances ease and flexibility ofpositioning of each of the first image capture device 32 and the secondimage capture device 34 respectively. The implementation of the tubelenses 36, 38 also eliminates the need to refocus illuminations enteringeach of the first image capture device 32 and the second image capturedevice 34 when different objective lenses are used. In addition, thecollimation of illumination increases ease of introduction andpositioning of additional optical components or accessories into thesystem 10, particularly between the objective lens 40 and each of thefirst tube lens 36 and the second tube lens 38. Further preferably, thecollimation of illumination enables in-situ introduction and positioningof additional optical components or accessories into the system 10,particularly between the objective lens 40 and each of the first tubelens 36 and the second tube lens 38, without a need for reconfiguringthe rest of the system 10. In addition, this arrangement helps toachieve longer working distance between objective lens 40 and the wafer12, compared to that of used in existing equipments. Longer workingdistances between the objective lens 40 and the wafer is necessary touse darkfield illuminations effectively.

It will therefore be appreciated by a person skilled in the art that thesystem 10 of the present invention allows for flexible and in-situdesign and reconfiguration of components of the system 10. The system 10of the present invention enhances ease of introduction and removal ofoptical components or accessories into and out of the system 10.

The first tube lens 36 facilitates focusing of collimated illuminationonto the first image capture plane. Similarly, the second tube lens 38facilitates focusing of collimated illumination onto the second imagecapture plane. Although, tube lenses are described for use with thesystem 10 in the present description, it will be appreciated by a personskilled in the art that alternative optical devices or mechanisms may beused for enabling collimation of illumination, more specifically thebrightfield illumination and darkfield illuminations, and the subsequentfocusing thereof onto either of the first image capture plane and thesecond image capture plane respectively.

The first image capture device 32 and the second image capture device 34are preferably positioned along adjacent parallel axes. Preferably, thespatial positions of the first image capture device 32 and the secondimage capture device 34 are determined for reducing space occupied bythe first image capture device 32 and the second image capture device 34such that the system 10 occupies a smaller total area (i.e. isspace-efficient).

Preferably, the system 10 further comprises a number of beam splittersand mirrors or reflective surfaces. The beam splitters and mirrors orreflective surfaces are preferably positioned for directing thebrightfield illumination and the darkfield illuminations from each ofthe low angle darkfield illuminator 28 and high angle darkfieldilluminator 30.

Preferably, the system 10 further comprises a central processing unit(CPU) with a storage memory or database (also known as a post processor)(not shown). The CPU is preferably electrically communicatable with orcoupled to the other components of the system 10, for example the firstimage capture device 32 and the second image capture device 34. Imagescaptured by the first image capture device 32 and the second imagecapture device 34 are preferably converted into image signals andtransmitted to the CPU.

The CPU is programmable for processing information, more specificallythe images, transmitted thereto to thereby detect defects present on thewafer 12. Preferably, the detection of defects on the wafer 12 isperformed automatically by the system 10, and more specifically by theCPU. Further preferably, the inspection of wafers 12 by the system 10 isautomatic, and controlled by the CPU. Alternatively, the inspection ofwafers 12 for the detection of defects is facilitated by at least onemanual input.

The CPU is programmable for storing information transmitted thereto in adatabase. In addition, the CPU is programmable for classifying detecteddefects. In addition, the CPU is preferably programmed for storingprocessed information, more specifically the processed images anddefects detected, in the database. Further details regarding capture ofimages, processing of captured images, and detection of defects on thewafers 12 are provided below.

It will be appreciated by a person skilled in the art, using thedescription provided above, that the brightfield illumination emittedfrom or supplied by the brightfield illuminator 26 and the darkfieldilluminations emitted from each of the low angle darkfield illuminator28 and the high angle darkfield illuminator 30 (hereinafter referred toas darkfield low angle or DLA illumination and darkfield high angle orDHA illumination respectively) each follows a different ray path oroptical path.

A flowchart of an exemplary first ray path 100 followed by thebrightfield illumination is shown in FIG. 10.

In a step 102 of the first ray path 100, brightfield illumination orlight is supplied by the brightfield illuminator 26. As previouslymentioned, the brightfield illumination is preferably emitted from thefirst optical fiber of the brightfield illuminator 26. Preferably, thefirst optical fiber directs the brightfield illumination emitted fromthe brightfield illuminator 26. The brightfield illumination preferablypasses through a condenser 44. The condenser 44 concentrates thebrightfield illumination.

In a step 104, the brightfield illumination is reflected by a firstreflecting surface 46 or a first mirror. Brightfield illuminationreflected by the first reflecting surface 46 is directed towards a firstbeam splitter 48.

The first beam splitter 48 reflects at least a portion of thebrightfield illumination striking thereonto in a step 106. Preferably,the first beam splitter 48 has a reflection/transmission (R/T) ratio of30:70. It will however be understood by a person skilled in the art thatthe R/T ratio of the first beam splitter 48 can be adjusted as requiredfor controlling the intensity or amount of brightfield illuminationreflected or transmitted thereby.

The brightfield illumination reflected by the first beam splitter 48 isdirected towards the inspection position. More specifically, thebrightfield illumination reflected by the first beam splitter 48 isdirected towards the objective lens 40 positioned directly above theinspection position. In a step 108, the brightfield illuminator 26 isfocused by the objective lens 40, at the inspection position or thewafer 12 positioned at the inspection position.

Brightfield illumination supplied by the brightfield illuminator 26, andfocused at the inspection position, illuminates the wafer 12, morespecifically the portion of the wafer 12, positioned at the inspectionposition. In a step 110, the brightfield illumination is reflected bythe wafer 12 positioned at the inspection position.

Brightfield illumination reflected by the wafer 12 passes through theobjective lens 40 in a step 112. As previously mentioned, the objectivelens 40 has a corrected aberration in infinity. Therefore, brightfieldillumination passing through the objective lens 40 is collimated by theobjective lens 40. The degree of magnification of the brightfieldillumination by the magnifying lens is dependent on the magnificationfactor of the objective lens 40.

Brightfield illumination passing through the objective lens 40 isdirected towards the first beam splitter 48. In a step 114, thebrightfield illumination strikes the first beam splitter 48 and aportion of thereof is transmitted through the first beam splitter 48.Extent of the brightfield illumination transmitted through the firstbeam splitter 48 in the step 114 depends on the R/T ratio of the firstbeam splitter 48. Brightfield illumination transmitted through the firstbeam splitter 48 travels towards a second beam splitter 50.

The second beam splitter 50 of the system 10 is preferably a cubic beamsplitter 50 having a predetermined R/T ratio. Preferably the R/T ratiois 50/50. The R/T ratio may be varied as required. The cubic beamsplitter 50 is preferred because the cubic bean splitter 50 splitsillumination received thereby into two optical paths. It will thereforebe appreciated by a person skilled in the art that the configuration andshape of the cubic beam splitter 50 will provide better performance andalignment for this purpose. Extent of illumination reflected ortransmitted by the second beam splitter 50 is dependent on the R/T ratioof the second beam splitter 50. In a step 116, the brightfieldillumination strikes the second beam splitter 50. The brightfieldillumination striking the beam splitter is either transmittedtherethrough or reflected thereby.

Brightfield illumination transmitted through the second beam splitter 50travels towards the first image capture device 32. The brightfieldillumination passes through the first tube lens 36 in a step 118 beforeentering the first image capture device 32 in a step 120. The first tubelens 36 helps to focus the collimated brightfield illumination onto thefirst image capture plane of the first image capture device 32.Brightfield illumination focused onto the first image capture planeenables capture of a brightfield image by the first image capture device32.

The brightfield image captured by the first image capture plane ispreferably converted into image signals. The image signals aresubsequently transmitted or downloaded to the CPU. The transmission ofimage signals to the CPU is also known as data transfer. Transferredbrightfield images are then at least one of processed by and stored inthe CPU.

Brightfield illumination reflected by the second beam splitter 50travels towards the second image capture device 34. The brightfieldillumination passes through the second tube lens 38 in a step 122 beforeentering the second image capture device 34 in a step 124. The secondtube lens 38 helps to focus the collimated brightfield illumination ontothe second image capture plane. Brightfield illumination focused ontothe second image capture plane enables capture of a brightfield image bythe second image capture device 34.

The brightfield image captured by the second image capture plane ispreferably converted into image signals. The image signals aresubsequently transmitted or downloaded to the CPU. The transmission ofimage signals to the programmable controller is also known as datatransfer. Transferred brightfield images are then at least one ofprocessed by and stored in the CPU.

A flowchart of an exemplary second ray path 200 followed by thedarkfield high angle (DHA) illumination is shown in FIG. 11.

In a step 202 of the second ray path 200, DHA illumination is suppliedby the high angle darkfield illuminator 30. As previously mentioned, thesecond optical fiber preferably helps to direct the DHA illuminationsupplied from the high angle darkfield illuminator 30. Preferably, theDHA illumination is directly focused at the inspection position withouta need to pass through optical components or accessories, for examplethe objective lens 40.

In a step 204, DHA illumination directed at the inspection position isreflected by the wafer 12, or the portion thereof, positioned at theinspection position. Reflected DHA illumination from the wafer 12 passesthrough the objective lens 40 in a step 206. The objective lens 40,which has a corrected aberration in infinity, collimates the DHAillumination passing therethrough in the step 206.

DHA illumination passing through the objective lens 40 is directedtowards the first beam splitter 48. In a step 208, the DHA illuminationstrikes the first beam splitter 48 and a portion thereof is transmittedthrough the first beam splitter 48. The extent of transmission of theDHA illumination through the first beam splitter 48 is dependent on theR/T ratio of the first beam splitter 48.

DHA illumination transmitted through the first beam splitter 48 isdirected towards the second beam splitter 50. In a step 210, the DHAillumination strikes the second beam splitter 50. Transmission orreflection of the DHA illumination striking the second beam splitter 50is dependent on the R/T ratio of the second beam splitter 50.

DHA illumination transmitted through the second beam splitter 50 passesthrough the first tube lens 36 in a step 212 before entering the firstimage capture device 32 in a step 214. The first tube lens 36 helps tofocus the collimated DHA illumination onto the first image capture planeof the first image capture device 32. DHA illumination focused onto thefirst image capture plane enables capture of a darkfield image, morespecifically a darkfield high angle (DHA) image by the first imagecapture device 32.

Alternatively, DHA illumination is reflected by the second beam splitter50. Reflected DHA illumination, from the second beam splitter 50, passesthrough the second tube lens 38 in a step 216 before entering the secondimage capture device 34 in a step 218. The second tube lens 38 helps tofocus the collimated DHA illumination onto the second image captureplane of the second image capture device 34. DHA illumination focusedonto the second image capture place enables capture of a darkfieldimage, more specifically a darkfield high angle (DHA) image by thesecond image capture device 34.

A flowchart of an exemplary third ray path 250 followed by the darkfieldlow angle (DLA) illumination is shown in FIG. 12

In a step 252 of the third ray path 200, DLA illumination is supplied bythe low angle darkfield illuminator 28. The third optical fiberpreferably helps to direct the DLA illumination supplied by the lowangle darkfield illuminator 28. Preferably, the DLA illumination isdirectly focused at the inspection position without a need to passthrough optical components or accessories, for example the objectivelens 40.

In a step 254, DLA illumination directed at the inspection position isreflected by the wafer 12, or the portion thereof, positioned at theinspection position. Reflected DLA illumination from the wafer passesthrough the objective lens 40 in a step 256. The objective lens 40,which has a corrected aberration in infinity, collimates the DLAillumination passing therethrough in the step 256.

DLA illumination passing through the objective lens 40 is directedtowards the first beam splitter 48. In a step 258, the DLA illuminationstrikes the first beam splitter 48 and a portion thereof is transmittedthrough the first beam splitter 48. The extent of transmission of theDLA illumination through the first beam splitter 48 is dependent on theR/T ratio of the first beam splitter 48.

DLA illumination transmitted through the first beam splitter 48 isdirected towards the second beam splitter 50. In a step 260, the DLAillumination strikes the second beam splitter 50. Transmission orreflection of the DLA illumination striking the second beam splitter 50is dependent on the R/T ratio of the second beam splitter 50.

DLA illumination transmitted through the second beam splitter 50 passesthrough the first tube lens 36 in a step 262 before entering the firstimage capture device 32 in a step 264. The first tube lens 36 helps tofocus the collimated DLA illumination onto the first image capture planeof the first image capture device 32. DLA illumination focused onto thefirst image capture plane enables capture of a darkfield image, morespecifically a darkfield high angle (DLA) image by the first imagecapture device 32.

Alternatively, DLA illumination is reflected by the second beam splitter50. Reflected DLA illumination from the second beam splitter 50, passesthrough the second tube lens 38 in a step 266 before entering the secondimage capture device 34 in a step 268. The second tube lens 38 helps tofocus the collimated DLA illumination onto the second image captureplane of the second image capture device 34. DLA illumination focusedonto the second image capture place enables capture of a darkfieldimage, more specifically a darkfield high angle (DLA) image by thesecond image capture device 34.

It will be appreciated by a person skilled in the art from thedescription provided above that the DHA illumination and DLAillumination preferably follows a similar ray path after being reflectedby the wafer 12. However, the second ray path 200 of the DHAillumination and the third ray path 250 of the DLA illumination can beindividually altered as required using techniques known in the art. Inaddition, the angles at which the DHA illumination and the DLAillumination strike at wafer 12 positioned at the inspection positionmay be adjusted as required for enhancing accuracy of defect detection.For example, the angles at which the DHA illumination and the DLAillumination strike at wafer 12 positioned at the inspection positionmay be adjusted depending on type of wafer 12 positioned at theinspection position or type of wafer defect that a user of the system 10wishes to detect.

The DHA images and the DLA images capture by each of the first imagecapture device 32 and the second image capture device 34 is preferablyconverted into image signal, which are subsequently transmitted ordownloaded to the CPU. The transmission of image signals to CPU is alsoknown as data transfer. Transferred DHA images and DLA images can thenbe at least one of processed by and stored in the CPU as required.

As previously mentioned, the first image capture device 32 and thesecond image capture device 34 have predetermined spatial positionsrelative each other. The use of the objective lens 40 together with thefirst tube lens 36 and the second tube lens 38 facilitates the spatialpositioning of the first image capture device 32 and the second imagecapture device 34. It will further be appreciated by a person skilled inthe art that other optical components or accessories, for exampleminors, may be used for directing the brightfield illumination, DHAillumination and DLA illumination, and for facilitating the spatialpositioning of the first image capture device 32 and the second imagecapture device 34. Preferably, the spatial positions of the first imagecapture device 32 and the second image capture device 34 are fixed withreference to the inspection position. The fixed spatial positions of thefirst image capture device 32 and the second image capture device 34preferably enhances at least one of the accuracy and the efficiency ofwafer inspection by the system 10. For example, the fixed spatialpositions of the first image capture device 32 and the second imagecapture device 34 with respect to the inspection position preferablyreduces calibration losses and adjustment feedback losses typicallyassociated with the use of mobile image capture devices or cameras.

The optical inspection head 14 of the system 10 preferably furthercomprises a third illuminator (hereinafter referred to as a thin lineilluminator 52). The thin line illuminator 52 is alternatively known asa thin line illumination emitter. The thin line illuminator 52 emits orsupplies thin line illumination. The thin line illuminator 52 ispreferably a laser source for supplying thin line laser illumination.More preferably, the thin line illuminator 52 is a broadband illuminatorsupplying a broadband thin line illumination. The thin line illuminationis preferably directed at the inspection position, more specifically atthe wafer 12 positioned at the inspection position, at a predeterminedangle, which can be varied as required. A mirror setup 54 or mirror ispreferably coupled to, or positioned at a predetermined positionrelative to, the thin line illuminator 52 for directing the thin lineillumination at the inspection position.

The optical inspection head 14 of the system 10 preferably comprises athird image capture device (hereinafter referred to as athree-dimensional (3D) profile camera 56). Preferably, the 3D profilecamera 56 receives the thin line illumination reflected by the wafer 12.Preferably, the thin line illumination entering the 3D profile camera 56is focused onto a 3D image capture plane (not shown) to thereby capture3D images of the wafer 12. The 3D optical setup comprising the thin lineilluminator 52 and the 3D profile camera 56 is shown in FIG. 13.

The optical inspection head 14 further comprises an objective lens forthe 3D profile camera 56 (hereinafter referred to as a 3D profileobjective lens 58). The thin line illumination reflected by the wafer 12passes through the 3D profile objective lens 58 before entering the 3Dprofile camera 56. Preferably, the 3D profile objective lens 58 has acorrected aberration in infinity. Accordingly, the thin lineillumination passing through the 3D profile objective lens 58 iscollimated thereby. The optical inspection head 14 further comprises atube lens 60 for use with the 3D profile objective lens 58 and the 3Dprofile camera 56. The tube lens 60 enables focusing of the collimatedthin line illumination onto the 3D image capture plane. The use of thetube lens 60 with the 3D profile objective lens 58 and the 3D profilecamera 56 facilitates flexible positioning and reconfiguration of the 3Dprofile camera 56. In addition, the use of the tube lens 60 with the 3Dprofile objective lens 58 and the 3D profile camera 56 enables ease ofintroducing additional optical components or accessories between the 3Dprofile objective lens 58 and the tube lens 60.

The thin line illuminator 52 and the 3D profile camera 56 preferablyoperate cooperatively for facilitating 3D profile scanning andinspection of the wafer 12. Preferably, the thin line illuminator 52 andthe 3D profile camera 56 are coupled to the CPU, which helps tocoordinate or synchronize the operation of the thin line illuminator 52and the 3D profile camera 56. Further preferably, an automated 3Dprofile scanning and inspection of the wafer 12 is performed by thesystem 10. This automated 3D profile scanning and inspection of thewafer 12 is preferably controlled by the CPU.

In addition, the optical inspection head 14 comprises a review imagecapture device 62. The review image capture device 62 is for example acolor camera. The review image capture device 62 preferably capturescolor images. Alternatively, the review image capture device 62 capturesmonochromatic images. The review image capture device 62 preferablycaptures review images of the wafer 12 for at least one of confirming,classifying and reviewing defect detected on the wafer 12.

The optical inspection head 14 further comprises a review brightfieldilluminator 62 and a review darkfield illuminator 64 for supplyingbrightfield illumination and darkfield illumination respectively. Thereview image capture device 60 receives the brightfield illumination andthe darkfield illumination supplied by the review brightfieldilluminator 62 and the review darkfield illuminator 64 respectively, andreflected by the wafer 12, for capturing review images of the wafer 12.Alternatively, the review image capture device 60 captures illuminationsupplied by alternative illuminators, for example one of that describedabove, for capturing review images of the wafer 12. The review imagecapture device 60 preferably captures high-resolution images of thewafer 12.

A diagram showing the review brightfield illuminator 62, the reviewdarkfield illuminator 64, the review image capture device 60 andillumination patterns therebetween, is provided in FIG. 14. A flowchartof an exemplary fourth ray path 300 followed by the brightfieldillumination supplied by the review brightfield illuminator 62 is shownin FIG. 15.

In a step 302 of the fourth ray path 300, brightfield illumination issupplied by the review brightfield illuminator 62. The brightfieldillumination supplied by the review brightfield illuminator 62 isdirected at a first reflective surface 66. In a step 304, thebrightfield illumination is reflected by the first reflective surface 66and directed towards a beam splitter 68. In a subsequent step 306, thebrightfield illumination striking the beam splitter 68 is reflectedthereby and directed towards the inspection position. Extent ofbrightfield illumination reflected by the beam splitter 68 depends onR/T ratio thereof.

In a step 308, the brightfield illumination is reflected by the wafer12, or portion thereof, positioned at the inspection position. Thereflected brightfield illumination passes through a review objectivelens 70 in a step 310. The review objective lens 70 may be integratedwith the objective lens mechanism or the objective lens assembly.Preferably, the review objective lens 70 has a corrected aberration ininfinity. Accordingly, the brightfield illumination passing through thereview objective lens 70 in the step 310 is collimated by the reviewobjective lens 70.

In a step 312, the brightfield illumination strikes the beam splitter 68and a portion thereof is transmitted therethrough. Extent of thebrightfield illumination passing through the beam splitter 68 isdepending on the R/T ratio of the beam splitter 68. The brightfieldillumination then passes through a review tube lens 72 in a step 314before entering the review image capture device 60 in a step 316. Thereview tube lens 72 focuses the collimated brightfield illumination ontoan image capture plane of the review image capture device 60.Brightfield illumination focused on the image capture plane of thereview image capture device 60 facilitates capture of review brightfieldimages in a step 318.

The collimation of the brightfield illumination between the reviewobjective lens 70 and the review tube lens 72 preferably facilitatesease of introduction of optical components and accessories therebetween.In addition, the collimation of the brightfield illumination between thereview objective lens 70 and the review tube lens 72 preferably enablesflexible positioning and reconfiguration as required of the review imagecapture device 60.

A flowchart of an exemplary fifth ray path 350 followed by the darkfieldillumination supplied by the review darkfield illuminator 64 is shown inFIG. 16.

In a step 352 of the fifth ray path 350, darkfield illumination issupplied by the review darkfield illuminator 64. The darkfieldillumination supplied by the review darkfield illuminator 64 ispreferably directly focused at the inspection position. In addition, thedarkfield illumination supplied by the review darkfield illuminator 64is preferably directed at the inspection position at a predeterminedangle to a horizontal plane of the wafer 12. This predetermined angle ispreferably a high angle, and can be adjusted as required usingtechniques known to a person skilled in the art.

In a step 354, the darkfield illumination is reflected by the wafer 12,or portion thereof, positioned at the inspection position. The reflecteddarkfield illumination then passes through the review objective lens 70in a step 356. The darkfield illumination passing through the reviewobjective lens 70 in the step 356 is collimated by the review objectivelens 70.

In a step 358, the collimated darkfield illumination strikes the beamsplitter and a portion thereof is transmitted therethrough. Extent ofthe darkfield illumination passing through the beam splitter 68 isdepending on the R/T ratio of the beam splitter 68. The darkfieldillumination then passes through the review tube lens 72 in a step 360before entering the review image capture device 60 in a step 362. Thefourth tube lens 72 focuses the collimated darkfield illumination ontoan image capture plane of the review image capture device 60. Darkfieldillumination focused on the image capture plane of the review imagecapture device 60 facilitates capture of review darkfield images in astep 364. The collimation of each of the brightfield illumination anddarkfield illumination between the review objective lens 70 and thereview tube lens 72 enhances ease of design and configuration of thesystem 10. More specifically, the collimation of each of the brightfieldillumination and darkfield illumination between the review objectivelens 70 and the review tube lens 72 enhances ease of positioning orconfiguration of the review image capture device 60 with the othercomponents of the system 10, thereby facilitating capture, while thewafer 12 is in motion, of the review brightfield images and reviewdarkfield images.

Captured review brightfield images and captured review darkfield imagesare preferably converted into image signals and transmitted from thereview image capture device 60 to the programmable controller where theycan be processed, and stored or saved in the database.

The review image capture device 60 can have a fixed spatial positionrelative the inspection position. The fixed spatial position of thereview image capture device 60 preferably reduces calibration losses andadjustment feedback losses typically associated with the use of mobileimage capture devices or cameras, thereby enhancing quality of reviewbrightfield images and review darkfield images captured.

The system 10 further comprises vibration isolators 24, which arecollectively known as a stabilizer mechanism. The system 10 ispreferably mounted on the vibration isolators 24 or stabilizer mechanismwhen the system is in normal operation. Preferably the system 10comprises four vibration isolators 24, each positioned at a differentcorner of the system 10. The vibration isolators 24 help to support andstabilize the system 10. Each vibration isolator 24 is preferably acompressible structure or canister, which absorbs ground vibrations tothereby serve as a buffer for preventing transmission of groundvibrations to the system 10. By preventing unwanted vibrations orphysical movements to the system 10, the vibration isolators 24 help toenhance quality of images captured by each of the first image capturedevice 32, the second image capture device 34, the 3D profile camera 56and the review camera 60, and to thereby improve quality of inspectionof the wafer 12.

An exemplary method 400 for inspecting the wafer 12 is providedaccording to an embodiment of the present invention. A method flowdiagram of the exemplary method 400 is shown in FIG. 17. The method 400for inspecting the wafer 12 enables at least one of detection,classification and review of defects on the wafer 12.

The exemplary method 400 for inspecting wafers 12 utilizes referenceimages (also known as golden references or golden reference images) towhich captured images of the wafers 12 are compared for at least one ofdetecting, classifying and review of defects on the wafers 12. Forpurposes of clarity, description of an exemplary reference imagecreation process 900 is provided before the description of the exemplarymethod 400 for inspecting wafers 12. The exemplary reference imagecreation process 900 is shown in FIG. 18.

Exemplary Reference Image Creation Process 900

In a step 902 of the reference image creation process 900, a recipecomprising a predetermined number of reference regions on the wafer 12is loaded. The recipe is preferably created or derived by a computersoftware program. Alternatively, the recipe is manually created. Therecipe can be stored in the database of the CPU. Alternatively, therecipe is stored in an external database or memory space.

Each of the predetermined number of reference regions representslocations on the wafer 12, which is of an unknown quality. The use ofmultiple reference regions helps to compensate for possibility ofsurface variations at different locations on the wafer 12, or betweenmultiple wafers 12. Such surface variations include, but are not limitedto, differential planarity and illumination reflectivity. It will beunderstood by a person skilled in the art that the predetermined numberof reference regions may represent an entire surface area of the wafer12. Alternatively, the predetermined number of reference regions mayrepresent multiple predetermined locations on multiple wafers.

In a step 904, a first reference region is selected. In a subsequentstep 906, a predetermined number (“n”) of images are captured of thefirst capture position of the selected reference region. Morespecifically, the n images are captured at each predetermined locationsof the selected reference region. Number and location of thepredetermined locations of the selected reference region can be variedas required and facilitated by at least one of software program andmanual input.

The n images can be captured using at least one of the first imagecapture device 32, the second image capture device 34, the 3D profilecamera 56 and the review image capture device 62 as required.Alternatively, the n images are captured using a different image capturedevice. Illuminations used for capture of the n images can be varied asrequired, and are for example one or a combination of the brightfieldillumination, the DHA illumination, the DLA illumination and the thinline illumination. Colors, wavelengths and intensities of theilluminations used for capture of the n images can be selected, andvaried, as required.

Capture of multiple images at each position preferably enables referenceimages to be created taking into account the variations in theillumination, optical setup and the imaging means used during capture ofthe reference images. This method of reference image creation minimizesunwanted influences or effects on defect detection, and classification,due to variations between the illumination conditions. In addition, anumber of images of the selected reference region may be captured foreach specified illumination condition. Preferably, capture of multipleimages at each specified illumination condition facilitates anormalizing or compensation of illumination variation from flash toflash or from strobe to strobe.

The n images are preferably stored in the database of the CPU.Alternatively, the n images are stored in an external database or memoryspace as required. In a step 908, the n images captured in the step 906are aligned and preprocessed. Preferably, subpixels of the n imagescaptured in the step 906 are registered. Registration of the subpixelsof the n images is preferably performed using known referencesincluding, but not limited to, traces, bumps or pads formed on the oneor more wafer using one or more of binary, grey scale or geometricalpattern matching.

In a step 910, reference intensities of each of the n images arecalculated. More specifically, reference intensity of each imagecaptured at each of the predetermined locations of the selectedreference region is calculated. Preferably, the calculation of referenceintensities of each of the n images helps to normalize or compensate forcolor variation at different locations or regions on the wafer 12 (orthe multiple wafers). Further preferably, the calculation of referenceintensities of each of the n images helps to account, or compensate, forother surface variations at different locations or regions on the wafer12 (or the multiple wafers).

The step 910 results in calculated of n reference intensities, each ofthe n reference intensities corresponding to one of the n images. In astep 912, a number of statistical information of intensities each pixelof each of the n images are calculated. The number of statisticalinformation includes, but is not limited to, an average, a range, astandard deviation, a maximum and a minimum intensity for each pixel ofeach of the n images.

More specifically, the average is a geometric mean of the referenceintensity for each pixel of each of the “n” images. Geometric mean is atype of mean or average, which indicates the central tendency or typicalvalue of a set of numbers, or n numbers. The numbers of the set aremultiplied and then the nth root of the resulting product is obtained. Aformula for obtaining geometric mean is shown below:

$\left( {\prod\limits_{i = 1}^{n}a_{i}} \right)^{1/n} = \sqrt[n]{{a_{1} \cdot a_{2}}\mspace{14mu}\ldots\mspace{14mu} a_{n}}$

Calculation of the geometric mean instead of arithmetic mean or medianprevents the average intensity calculated for each pixel of each of then images from being unduly affected by extreme values in a data set.

In addition, range of absolute intensity (hereinafter referred to as Ri)for each pixel of the n images is calculated. Preferably, the Ri foreach pixel of the “n” images is the value between a maximum and aminimum absolute intensity for each pixel of the n images.

As previously mentioned, the standard deviation of the intensity of eachpixel for each of the n images of the first reference region captured inthe step 906 is also calculated. More specifically, the standarddeviation is a geometric standard deviation, which describes how spreadout is a set of numbers whose preferred average is the geometric mean. Aformula for obtaining the standard deviation is shown below:

$\sigma_{g} = {{\exp\left( \sqrt{\frac{{\sum\limits_{i = 1}^{n}\left( {{\ln\; A_{i}} - {\ln\;\mu_{g}}} \right)^{2}}\;}{n}} \right)}.}$where μ_(g), is the geometric mean of a set of numbers {A₁, A₂, . . . ,A_(n)}.

In a step 914, the n images captured are temporarily saved, togetherwith their corresponding information such as location on the wafer 12 orfirst reference region. The statistical information calculated in thestep 912 is preferably also temporarily saved in the step 914.Preferably, the above data is saved in the database of the CPU.Alternatively, the above data is saved in an alternative database ormemory space as required.

In a step 916, it is determined if more images of the selected referenceregion are required. The step 916 is preferably software controlled andpreformed automatically. Preferably, the step 916 is performed with areliance on information obtained by the steps 910 and 912.Alternatively, the step 916 is manually facilitated or controlled usingtechniques known in the art.

If it is determined in the step 916 that more images of the selectedreference region are required, the steps 904 to 916 are repeated. Thesteps 904 to 916 can be repeated any number of times as required. Whenit is determined in the step 916 that no more images of the firstreference region is required, a step 918 of determining if the steps 904to 916 need to be repeated for a next reference region (for purposes ofthe present description, a second reference region) of the predeterminednumber of reference regions. The step 918 is preferably softwarecontrolled and performed automatically. In addition, the step 918 ispreferably performed using information obtained in at least one of steps910, 912 and 916. Alternatively, the step 918 is manually facilitated orcontrolled using alternative techniques known in the art.

If it is determined in the step 918 that images of the second referenceregion need to be captured, i.e. if the steps 904 to 916 need to berepeated for the second reference region, a signal is generated forrepeating the steps 904 to 916. The steps 904 to 918 can be repeated anynumber of times as required. Repetition of the steps 904 to 918 ispreferably software controlled and automated.

When it is determined in the step 918 that the steps 904 to 918 do notneed to be repeated, i.e. that images of the next reference region ofthe predetermined number of reference regions are not required, goldenreference images (hereinafter referred to as reference images) are thencalculated in a step 920.

The calculation or derivation of the reference images is preferablysoftware controlled, and is performed via a series of programinstructions. The following steps are exemplary steps performed forcalculating the reference images. It will however be understood by aperson skilled in the art that additional steps or techniquescomplementary to the following steps may be performed in the calculationof the reference image.

In a step 922, pixels having reference intensities greater than apredefined limit is determined. In addition, pixels having range ofpixel intensities greater than a predefined range is determined in thestep 922. The predefined limit and range of the step 922 can be softwareselected and determined or manually determined. In a step 924, pixels ofintensities with a standard deviation greater than a predefined valueare identified. The predefined value of the step 924 can be softwareselected and determined or manually determined. In a step 926, thepreviously saved images, as in the step 914, are reloaded for repeat ofany one of more of the steps 904 to 924 if a pixel with referenceintensities outside predetermined value or range is identified duringthe steps 922 to 924.

The steps 922 to 926 facilitate identification of images comprisingpixels of specific pixel intensities. More specifically, the steps 922to 926 enable identification of images containing pixels havingreference intensities outside predefined limits or ranges, for exampleidentification of “undesirable” images, to be identified. Morespecifically, the steps 922 to 926 eliminate “undesirable” pixels fromthe reference image calculation and help to prevent “undesirable” pixelsinfluence on the final reference pixel values of the reference image.

The “undesirable” images are discarded. This facilitates elimination ofdefective data or images, thereby preventing influence or presence ofsuch defective data with generated reference images. In a step 928,images comprising pixels within predefined limits and ranges (i.e.images not discarded) are consolidated.

Preferably, the reference image creation process 900 results inderivation of the following image data:

-   -   (a) Normalized average of intensity of each pixel of each of the        consolidated images    -   (b) Standard deviation of intensity of each pixel of each of the        consolidated images    -   (c) Maximum and minimum intensities of each pixel of each of the        consolidated images    -   (d) Average reference intensity of each of the predetermined        number of reference regions determined in the step 702

The consolidated images of the step 928 represent reference images. Thereference images, together with corresponding image data is furthersaved in the step 928. The reference images and their correspondingimage data are preferably saved in the database of the CPU.Alternatively, the reference images and their corresponding image dataare saved in an alternative database or memory space. It will beappreciated by a person skilled in the art that the step 922 to 926helps to reduce amount or size of memory space required for storing thereference images and their corresponding data, which may enable themethod 400 to be performed at a higher speed or accuracy.

The average intensity of each pixel is preferably normalized to 255 inorder to display and visualize the reference images. It will however beunderstood by a person skilled in the art that the average intensity ofeach pixel can be normalized to an alternative value in order to displayand visualize the reference images.

The steps 904 to 928 can be repeated a predetermined number of times forcapturing a corresponding number of images with at least one of thefirst image capture device 32, the second image capture device 34 andthe review camera. In addition, the steps 904 to 928 can be repeated forcapturing images at different illuminations or illumination conditions,for example brightfield illumination, DHA illumination, DLA illuminationand thin line illumination, as required. The repetition of the steps 904to 928 enables creation of reference images for multiple illuminationsor illumination conditions, and with multiple image capture devices asrequired.

As previously described, the derivation of reference images for multiplereference regions of the wafer 12 (or multiple wafer) and at multipleillumination conditions helps to ensure accountability, and compensationwhere required, for variations in quality of subsequently capturedimages due to variations in the lighting conditions. For example, thecapture of reference images at different reference regions of the wafer12 (i.e. different locations on the wafer 12) preferably ensuresaccountability and compensation for color variations at differentlocations on the wafer 12.

The steps 904 to 928 are preferably executed and controlled by the CPU.Preferably, the steps 904 to 928 are at least one of executed andcontrolled by a software program. Alternatively, at least one of thesteps 904 to 928 may be manually assisted if required. The referenceimages created by the exemplary reference image creation process 900 areused for comparison with subsequently captured images of wafers 12 ofunknown quality to thereby enable at least one of detection,classification and review of defects on the wafer 12.

As previously mentioned, the present invention provides the exemplarymethod 400 for inspection of wafers 12 to thereby at least one ofdetect, classify and review defects present on the wafers 12.

In a step 402 of the method 400, the wafer 12 to be inspected by thesystem 10 is loaded onto the wafer table 16. Preferably, the wafer 12 isextracted from the wafer stack 20 by the robotic wafer handler 18 andtransferred onto the wafer table 16. Suction or vacuum is applied to thewafer table 16 to secure the wafer 12 thereonto.

The wafer 12 preferably comprises a wafer identification number (IDnumber) or barcode. The wafer ID number or barcode is engraved or taggedonto a surface of the wafer 12, more specifically at a periphery of thesurface of the wafer 12. The wafer ID number or barcode helps toidentify the wafer 12 and ensures that the wafer 12 is correctly orappropriately loaded onto the wafer table 16.

In a step 404, a wafer map of the wafer 12 loaded onto the wafer table16 is obtained. The wafer map may be loaded from the database of theprogrammable controller. Alternatively, the wafer map may be retrievedfrom an external database or processor. Further alternatively, the wafermap may be prepared or derived upon the loading of the wafer 12 onto themovable support platform using methods or techniques known to a personskilled in the art.

In a step 406, one or more reference locations are captured ordetermined on the wafer map and at least one of wafer X, Y translationaland θ rotational offset is calculated using techniques known to a personskilled in the art.

In a subsequent step 408, a wafer scan motion path and a plurality ofimage capture positions are calculated or determined. The wafer mapobtained in the step 404 preferably facilitates the calculation of thewafer scan motion path and the plurality of image capture positions.Preferably, the calculation of the wafer scan motion path is dependenton at least one of several known parameters. Such known parametersinclude, but are not limited to, rotation offset, wafer size, wafer diesize, wafer pitch, inspection area, wafer scan velocity and encoderposition. Each of the plurality of image capture positions reflects orcorresponds to a position on the wafer 12 of which images are to becaptured. Preferably, each of the plurality of image capture positionscan be altered as required using techniques known to a person skilled inthe art. The number of image capture positions can also be altered asrequired using techniques known to a person skilled in the art.

Preferably, the steps 404 to 408 are performed automatically by thesystem 10, more specifically by the programmable controller of thesystem 10. Alternatively, any one of the steps 404 to 408 may beperformed by, or with the aid of, an alternative processor.

In a step 410, the programmable controller of the system 10 determinesavailability of an appropriate golden reference (hereinafter referred toas a reference image). If the reference image is not available, thereference image is created by the exemplary reference image creationprocess 900 as described above in a step 412.

Preferably, the reference image is obtained, or created, beforeperforming an exemplary two-dimensional (2D) wafer scanning process 400in a step 414. A process flow diagram of the exemplary two-dimensional(2D) wafer scanning process 500 is shown in FIG. 19.

Exemplary Two-Dimensional (2D) Wafer Scanning Process 500

The 2D wafer scanning process 500 enables capture of brightfield imagesand darkfield images by the first image capture device 32 and the secondimage capture device 34.

In a step 502 of 2D wafer scanning process 500, the first image capturedevice 32 is exposed. In a step 504, a first illumination is supplied oremitted. The first illumination is for example brightfield illuminationsupplied by or emitted from the brightfield illuminator 26, DHAillumination supplied by or emitted from the high angle darkfieldilluminator 30 or DLA illumination supplied by or emitted from the lowangle darkfield illuminator 28. Selection of the first illumination tobe supplied or emitted in the step 504 is preferably determined by anillumination configurator (not shown). Preferably, the illuminationconfigurator is a component of the system 10 and electronically coupledto the illuminators (28, 30, 52, 64 and 66) of the system 10.Alternatively, the illumination configurator is a component of the CPU.

The image capture devices 32 and 34 can receive or capture anycombination of illuminations supplied by or emitted from the brightfieldilluminator 26, DHA illuminator 30 and DLA illuminator 28. Examples ofpossible combinations for the first illumination received by the imagecapture device 32 and the second illumination received by the imagecapture device 34 are shown in the table of FIG. 20. If the first imagecapture device 32 and the second image capture device 34 receivesubstantially similar illuminations, then the throughput of such aconfiguration could be the highest of all the possible configurations.

For example, configuration 1 as shown in the table of FIG. 20 can beselected by the illumination configurator. Accordingly, the firstillumination is the brightfield illumination supplied by the brightfieldilluminator 26.

Preferably, the steps 502 and 504 are performed simultaneously.Performance of the steps 502 and 504 enables capture of a first image,as shown in FIG. 22a , by the first image capture device 32. In a step506, the first image captured by the first image capture device 32 isconverted into image signals and transmitted to the CPU via the datatransfer process and preferably stored in the database or storagememory.

In a step 508, the second image capture device 34 is exposed. In a step510, a second illumination is supplied. As with the first illumination,selection of the second illumination is preferably determined by theillumination configurator. For purposes of the present description,configuration 1 as shown in the table of FIG. 20 is selected by theillumination configurator. Accordingly, the second illumination is theDHA illumination supplied by the high angle darkfield illuminator 30. Itwill however be appreciated by a person skilled in the art that thefirst illumination and the second illuminations may be alternativeilluminations as required, for example the exemplary illuminations ofthe different configurations as shown in the table of FIG. 20.

Preferably, the steps 508 and 510 are performed simultaneously.Preferably, the step 506 occurs in tandem with the performance of thesteps 508 and 510. Performance of the steps 508 and 510 enables captureof a second image, as shown in FIG. 22b , by the second image capturedevice 34. In a step 512, the second image captured by the second imagecapture device 34 is converted into image signals and transmitted to theprogrammable controller via the data transfer process and preferablystored in the database or storage memory.

A diagram showing the exposure of the first image capture device 32,supply of the first illumination, exposure of the second image capturedevice 34, supply of the second illumination and the data transferprocess of each of the first image capture device 32 and the secondimage capture device 34 is provided in FIG. 21. The steps 502 to 512 canbe repeated any number of times for capturing a corresponding number ofsets of first images and second images of the wafer 12. Morespecifically, the steps 502 to 512 are preferably repeated for capturingimages of the wafer 12 using the first illumination and the secondillumination at each of the plurality of image capture positions alongthe wafer scan motion path as calculated in the step 408.

As previously described, each of the first image and the second imageare converted into image signals and transmitted to the programmablecontroller and stored in the database or storage memory. Each of thesteps 502 to 512 is performed while the wafer 12 is in motion. This isto say, the capture of the first image and the second image is performedwhile the wafer 12 is in motion along the wafer scan motion path.Accordingly, a person skilled in the art will appreciate that the wafer12 will be displaced by a predetermined distance along the wafer scanmotion path between the steps 502, 504 (which preferably occursimultaneously) and the steps 508, 510 (which preferably also occursimultaneously). The predetermined distance depends on several factorsincluding, but not limited to, speed of displacement of the wafer 12along the wafer scan motion path and time required for any one of thesteps 502 to 512. The predetermined distance may be controlled andvaried as required, for example by the CPU. The control and variation ofthe predetermined distance may be at least one of software or manuallyfacilitated.

Accordingly, the first image will have a predetermined image offset whensuperimposed onto or compared with the second image. FIG. 22c shows acombined image of the first image and the second image demonstratingresulting image offset due to the capture of the first image and thesecond image while the wafer 12 is in motion. The predetermined imageoffset depends on several factors including, but not limited to, speedof displacement of the wafer 12 along the wafer scan motion path andtime required for any one of the steps 502 to 512. Control and variationof the predetermined image offset may be at least one of software ormanually facilitated.

In a step 514, XY encoder values are retrieved. The XY encoder valuesare preferably obtained at during each of the steps 504 and 510.Preferably, the XY encoder values represent positions (XY-displacement)of the wafer 12 along the wafer scan motion path. The XY encoder valuesobtained are used for calculating the image offset (coarse offset)between the first image and the second image (i.e. relative offset ofthe second image from the first image) in a step 516. The fine imageoffset is calculated by performing sub pixel image alignment usingpattern matching techniques. The final offset is obtained by applying apredetermined mathematical formula on the coarse and fine image offsets.The predetermined mathematical formula may be adjusted as required usingtechniques known to a person skilled in the art.

The 2D wafer scanning process 500 performed in the step 414 of themethod 400 results in the capture of multiple images of the wafer 12,preferably at the calculated image capture positions along the waferscan motion path.

In a step 416 of the method 400, an exemplary two-dimensional (2D) imageprocessing process 600 is performed for at least one of identifying ordetecting, classifying, consolidating and storing defects on the wafer12. A process flow diagram of the exemplary 2D image processing process600 is shown in FIG. 23.

Exemplary 2D Image Processing Process 600

The 2D image processing process 600 facilitates processing of the imagescaptured in the 2D wafer scanning process 500. In addition, the 2D imageprocessing process 600 facilitates at least one of identifying ordetecting, classifying, consolidating and storing defects on the wafer12.

In a step 602 of 2D image processing process 600, a first working imageis selected and loaded in a memory workspace. The first working image isselected from the number of first images and second images captured andsaved during the 2D wafer scanning process. For purposes of the presentdescription, the first working image represents the first image capturedby the first image capture device 32 during the 2D wafer scanningprocess 500.

In a step 604, sub-pixel alignment of the first working image isperformed. Sub-pixel alignment is performed using pattern matchingtechniques using one or more templates. It is performed using one ofbinary or grey scale or geometrical pattern matching methods. Oncealigned, reference intensity for each image is calculated from one ormore predefined region of interests in the image as shown in a step 606.The reference intensity of each pixel of each image of wafer capturedalong the wafer scan motion path is calculated in the step 606. Thesteps 604 and 606 may be collectively referred to as a preprocessing ofthe first working image. It can be readily appreciated that thepreprocessing is not limited to above steps. Additional steps can beincorporated for the preprocessing of the first working image asrequired.

In a subsequent step 608, a first golden reference or a first referenceimage is selected. The first reference image selected in the step 608corresponds or matches with predetermined parameters or features of thefirst working image. Preferably, the first reference image is selectedfrom a database or collection of golden references or reference imagesas created by the exemplary reference creation process 900 in the step412 of the method 400. The exemplary reference creation process 900 isdescribed in detail above, and shown in FIG. 18.

In a step 610, quantitative data values for each pixel of the firstworking image are calculated. In a step 612, the calculated quantitativedata values for each pixel of the first working image are referencedwith a predetermined threshold value together with multiplicative oradditive factors.

In a step 614, the first working image is then matched or evaluatedagainst the first reference image selected in the 608. The matching orevaluation of the first working image with the first reference imagefacilitates detection or identification of defects on the wafer 12.Preferably, the CPU is programmed for effecting automated matchingbetween the first working image and the first reference image. Theprogrammable controller preferably carries out a series of computinginstructions or algorithms for matching the first working image with thefirst reference image to thereby enable the detection or identificationof defects on the wafer 12.

Determination of presence of one or more defects occurs in a step 616 ofthe 2D image processing process 600. If more than one defects aredetected or identified in the step 616, the algorithm would sort thedefects from the largest to the shortest based on either one or all ofarea, length, width, contrast, compactness, fill factor, edge strengthamong others. Furthermore, the algorithm selects only those defectswhich meets user defined or predetermined criteria or parameters tocalculate defective region of interest (DROI) on the wafer in a step618. This is to say, DROI on the wafer 12 is calculated in the step 618if a defect (or more than one defects) is detected or identified in thestep 616. Preferably, the DROI is calculated dynamically by the CPU inthe step 618. The CPU is preferably programmed (i.e. comprises orembodies a series of computing instructions or software) for enablingthe calculation of the DROI.

In a step 620, a corresponding DROI of a second working image isinspected. More specifically, the second working image is the secondimage captured by the second image capture device 34 during the 2D waferscanning process 400. The second working image is preferablypreprocessed as according to the steps 604 and 606 before performing thestep 620. This is to say, the DROI of the second working image (thefirst working image and the second working image being captured of thesame location on the wafer 12 along the wafer scan motion path), isinspected in the step 620 after performing sub-pixel alignment of secondworking image. The inspection of the DROI of the second working imagepreferably facilitates confirmation of defect detected in the step 616.Further preferably, the step 620 facilitates classification of defectdetected in the step 606.

The system 10 processes the DROIs of the second working image instead ofthe entire image. In addition, in the step 616, if there is no defectfound, the method then terminates at the step 616 (i.e. methods stepsbeyond 616 will not be performed). This will further reduce the amountresources or processing bandwidth needed for processing the secondworking image. It can be readily appreciated that such an intelligentprocessing sequence (i.e. flow of steps in the method) is dynamicallydecided or performed based on results of preceding steps of the method.The intelligent processing of the 2D image processing process 600facilitates improvement of the system's 10 throughput (i.e. wafersinspected per hour by the system 10).

In a step 622, the detected defect, more specifically the location orposition of the detected defect as well as the classification thereof,is saved. Preferably, the detected defect, and the location andclassification thereof, is saved in the database of the CPU.Alternatively, the detected defect, and the location and classificationthereof, is saved in an alternative database or memory space.

The steps 602 to 622 can be repeated or looped any number of times forprocessing the images captured during the 2D wafer scanning process 500.Each of the images captured during the 2D wafer scanning process 500 issequentially loaded in the memory workspace and processed forfacilitating the detection of defects, which may be present on the wafer12. The steps 602 to 622, and the repetition thereof, facilitates atleast one of detection, confirmation, and classification of defects,which may be present on the wafer 12 at any of the multiple imagecapture positions along the wafer scan motion path.

In a step 624, each of multiple defects, and the locations andclassifications thereof, detected by the 2D image processing process 600are consolidated and saved, preferably in the database of the CPU.Alternatively the defects, and the locations and classificationsthereof, are consolidated and saved in an alternative database or memoryspace.

The 2D image processing process is preferably an automated process.Preferably, the CPU is programmed for, or comprises a series ofcomputing instructions or software program, for automatically performingthe 2D image processing process. Alternatively, the 2D image processingprocess may be facilitated by an at least one manual input as required.

Completion of the 2D image processing process 600 of the step 416 of themethod 400 results in consolidation and storage of defects, and thelocations and classifications thereof, detected using the brightfieldillumination, the DHA illumination and DLA illumination.

In a subsequent step 418 of the method 400, a first exemplarythree-dimensional (3D) wafer scanning process 700 is performed.Preferably, the first 3D wafer scanning process 700 enables capture of3D profile images of the wafer 12, for facilitating consequent formationof a 3D profile of the wafer 12. The wafer 12 is displaced along thecalculated wafer scan motion path for capturing 3D images of the wafer12 at any one or more of the multiple images capture positions along thewafer scan motion path as calculated in the step 408. A process flowdiagram of the first exemplary 3D wafer scanning process 700 is shown inFIG. 24.

Exemplary 3D Wafer Scanning Process 700

In a step 702 of the 3D wafer scanning process, thin line illuminationis supplied by or emitted from the thin line illuminator 52. In a step704, the thin line illumination is directed at the inspection positionby the mirror setup 54.

In a subsequent step 706, the thin line illumination is reflected by thewafer 12, or portion thereof, positioned at the inspection position.Reflected thin line illumination from the wafer 12 is transmittedthrough the 3D profile objective lens 58 in a step 708. The 3D profileobjective lens 58 has aberration corrected in infinity. Transmission ofthe thin line illumination through the 3D profile objective lens 58 inthe step 708 collimates the thin line illumination.

In a step 710, the collimated thin line illumination then passes throughthe tube lens 60 before entering the 3D profile camera 56 in a step 712.The tube lens 60 preferably focuses the collimated thin lineillumination onto the image capture plane of the 3D profile camera 56.Thin line illumination focused on the 3D image capture plane enablescapture of a first 3D image of the wafer 12 in a step 714. Collimationof the thin line illumination between the 3D profile objective lens 58and the tube lens 60 facilitates ease of introduction of opticalcomponents or accessories therebetween, and enable flexible positioningand reconfiguration of the 3D profile camera 56.

As previously mentioned, the thin line illumination is supplied by alaser or broadband fiber optic illumination source. In addition, thethin line illumination is preferably directed at the inspection positionat a specified angle with reference to a horizontal plane of the wafer12 positioned thereat. The angle at which the thin line illumination isdirected at the inspection position is preferably variable as requiredusing techniques known to a person skilled in the art. It will also beappreciated by a person skilled in the art that the wavelength of thethin line illumination may be selected, and varied, as required.Preferably, broadband wavelength of the thin line illumination isselected for enhancing accuracy of at least one of defect detection,verification, and classification. The wavelength of the thin lineillumination is preferably similar to that of at least one of thebrightfield illumination, the DHA illumination and the DLA illumination.

The first 3D image is converted to image signals and transmitted to theCPU in a step 716. In a step 718, the first 3D image is processed by theCPU for at least one of 3D height measuring, coplanarity measuring,detecting and classifying a defect.

Preferably the steps 702 to 718 can be repeated any number of times forcapturing a corresponding number of 3D images, transmitting thecorresponding number of captured 3D images to the CPU, and processingthe corresponding number of 3D images. The steps 702 to 718 can beperformed at any number of predetermined or selected image capturepositions along the wafer scan motion path of the wafer 12.

Preferably, the first 3D wafer scanning process 700 enhances accuracywith which the exemplary method 400 inspects a semiconductor wafer. Morespecifically, the first 3D wafer scanning process 700 enhances accuracyof defect detection by the method 400. The 3D wafer scanning process 700provides details 3D meteorological details such coplanarity, height ofthree-dimensional structures such as solder balls, gold bumps, warpageof individual die of the wafer 12 as well as of the entire wafer 12.

Preferably, results of the step 718, and repetitions thereof, ofprocessing of 3D images are saved in the database of the CPU.Alternatively, results of the step 718, and repetitions thereof, ofprocessing of 3D images are saved in an alternative database or memoryspace as required.

An exemplary second three-dimensional (3D) wafer scanning process 750can also be used instead of the first exemplary 3D wafer scanningprocess 700. The optical ray path of the exemplary second 3D waferscanning process 750 is shown in FIG. 25 and a corresponding processflow diagram of the exemplary second 3D wafer scanning process 750 is asshown in FIG. 26.

In a step 752 of the second 3D wafer scanning process 750, thin lineillumination is supplied by or emitted from the thin line illuminator52. In a step 754, the thin line illumination is directed at theinspection position by a reflector assembly 80. The reflector assembly80 is alternatively known as a prism assembly or a two mirror or prismsetup.

In a step 756, the thin line illumination is reflected by the wafer 12.Thin line illumination reflected by the wafer 12 can be reflected indifferent directions depending on the surface profile of the wafer 12.For example, structural and geometrical variations on the wafer 12 cancause the thin line illumination to be reflected by the wafer 12 indifferent directions (otherwise known as dispersion of illumination).

Thin line illumination reflected by the wafer 12 is received by thereflector assembly 80. More specifically, the reflector assembly 80 isconfigured for capturing thin line illumination reflected in multipledirections. Preferably, the reflector assembly 80 comprises a first pairof mirrors or prisms 82 and a second pair of mirrors or prisms 84. In astep 758, reflected thin line illumination travels along two opticalpaths, namely a first optical path via, or as directed by, the firstpair of mirrors or prisms 82 and a second optical path via, or asdirected by, the second pair of mirrors or prisms 84. It will beunderstood by a person skilled in the art that the reflector assemblycan be configured for directing captured reflected thin lineillumination along a different number optical paths as required.

The thin line illumination traveling along each of the first opticalpath and the second optical path passes through the objective lens 58 ina step 760. The two thin line illuminations passing through the 3Dprofile objective lens 58 are collimated. The first pair of mirrors orprisms 82 and the second pair of mirrors or prisms 84 are preferablysymmetrically positioned.

In a step 762, the two collimated thin line illuminations passes throughthe tube lens 60. The two thin line illuminations then enters the 3Dprofile camera 56 in a step 764. The tube lens 60 facilitates focusingof the two thin line illuminations onto the image capture plane of the3D profile camera 56. Focusing of the two thin line illuminations ontothe image capture plane of the 3D profile camera 56 enables capture amultiple view 3D profile image of the wafer 12 in a step 766.

The multiple view 3D profile image of the wafer 12 is converted to imagesignals and transmitted to the CPU in a step 768. In a step 770, themultiple view 3D profile image is processed by the CPU for at least oneof 3D height measuring, coplanarity measuring, detecting and classifyinga defect. Preferably the steps 752 to 770 can be repeated any number oftimes for capturing a corresponding number of multiple view 3D images,transmitting the corresponding number of captured multiple view 3Dimages to the CPU, and processing the corresponding number of multipleview 3D images.

The ability of the system 10 to perform the second 3D wafer scanningprocess 750 enables capture of multiple view 3D images of the wafer 12using a single 3D image capture device 56. In other words, the second 3Dwafer scanning process 750 enables capture of images having multipleviews of the wafer 12. Each of the multiple views of the multiple view3D images captured represents illumination reflected by the wafer 12 ina different direction. Capture of multiple view 3D images of the wafer12 (i.e. 3D images having multiple views of the wafer 12) improveaccuracy of the 3D profiling or inspection of the wafer 12. In addition,the use of the two symmetrically positioned mirrors or prisms 82 and 84enables illumination reflected from the wafer 12 in different directionsto be re-directed for capture by the 3D image capture device 56. It willbe understood by a person skilled in the art that the reflector assembly80 can be configured for directing illumination reflected from the wafer12 in multiple directions (for example, two, three, four and fivedirections) to be captured by a single exposure of the 3D image capturedevice 56.

To receive two views of the same profile of wafers 12, existingequipments use expensive, bulky and complicated setup by using multipleimage capture devices. Due to the inconsistent profile of wafers 12,reflected rays do not consistently return in a predetermined ray path tothe multiple image capture devices. This is to say, the dispersion ofillumination due to structural and geometrical variations on the surfaceof the wafer 12 typically results in inaccuracies in single view imagescaptured of the wafer 12.

To overcome the variation of the strength and weakness (i.e. thedispersion) of the reflected rays from the wafers, the present system 10enables illumination reflected from the wafer 12 in different directionsto be captured by the 3D image capture device 56. More specifically, thesystem 10 utilizes the reflector assembly 80 for receiving and directingthe illumination reflected by the wafer 12 in different directions forsubsequent collective capture by the 3D image capture device 56. Thishelps to improve accuracy of 3D profiling and inspection of the wafer12. The use of a single camera, more specifically the 3D image capturedevice 56, also enhances cost and space efficiency of the system 10.Furthermore, the ability to use a single objective lens and a singletube lens (in this case, the objective lens 58 and tube lens 60) forcapturing multiple views of the wafer 12 enhances ease and accuracy ofcalibration.

After completion of the first 3D wafer scanning process 700 or thesecond 3D wafer scanning process 750, all the detected defects, and thelocations and classifications thereof, on the wafer 12 obtained byperforming the steps 416 and 418 are preferably consolidated. Theconsolidation of the defects, and the locations and classificationsthereof, facilitates calculation of a review scan motion path in a step420. Preferably, the review scan motion path is calculated based on thelocations of defects detected on the wafer 12 along the wafer scanmotion path. In addition, defect image capture positions along thereview scan motion path is calculated or determined in the step 420. Thedefect image capture positions as calculated in the step 420 preferablycorrespond with the locations on the wafer 12 at which defects weredetected (i.e. the DROI of the wafer 12) during the steps 416 and 418.

In a step 422 of the exemplary method 400, an exemplary review process800 is performed. The review process 800 enables review of defectsdetected in the steps 416 and 418. Preferably, the review process 800occurs via at least one of a first mode 800 a, a second mode 800 b and athird mode 800 c. A process flow chart of the exemplary review process800 is shown in FIG. 27.

Exemplary Review Process 800

As previously mentioned, the review process 800 preferably comprisesthree review modes, namely the first mode 800 a, the second mode 800 band the third mode 800 c. In a step 802, a review mode (i.e. one thefirst mode 800 a, the second mode 800 b and the third mode 800 c) isselected.

First Mode 800 a of the Review Process 800

In a step 804 of the first mode 800 a of the review process 800, thefirst images and the second images of all the defects detected duringthe 2D image processing process 600 performed in the step 416 of themethod 400 are consolidated and saved.

In a step 806, the consolidated and saved first images and second imagesof the defects detected on the wafer 12 are uploaded or transferred toan external storage or server for an offline review.

In a step 808, the wafer 12 (i.e. the current wafer 12 on the wafertable 16) is unloaded and a second wafer is loaded from the wafer stack20 onto the wafer table 16 by the robotic arm. In a step 810, each ofthe steps 804 to 808 is repeated for the second wafer.

The steps 804 to 810 are sequentially repeated any number of times,depending on the number of wafers of the wafer stack 20. Repetition ofthe step 804 to 810 results in consolidation and saving of the firstimages and second images obtained for each wafer of the wafer stack 20,and the uploading of the first images and the second images, to theexternal storage or server for an offline review. It will be appreciatedby a person skilled in the art that the first mode 800 a of the reviewprocess 800 enables automated performance of the steps 804 to 810without need for user intervention and without affecting production. Thefirst mode 800 a of the review process 800 allows continuing productionwhile user can perform offline review of saved images. In addition, thefirst mode 800 a of the review process 800 increases system 10utilization as well as productivity.

Second Mode 800 b of the Review Process 800

In a step 820 the second mode 800 b of the review process 800, a numberof review images is captured at each of the defect image capturepositions as calculated in the step 420. More specifically, a reviewbrightfield image and a review darkfield image are captured at each ofthe defect image capture positions as calculated in the step 420 usingreview image capture device 60 shown in FIG. 14. This is to say a reviewbrightfield image using brightfield illuminator 62 and a reviewdarkfield image using the darkfield illuminator 64 are captured of eachdefect as detected or identified by the 2D image processing process 600of the step 416. Each of the number of review images is captured by thereview image capture device 60. Preferably, each of the number of reviewimages is a colored image.

It will be understood by a person skilled in the art, provided with thedisclosure of the present description that intensities of thebrightfield illumination and darkfield illumination used for capturingthe brightfield review images and darkfield review images respectivelymay be determined and varied as required. For example, the intensitiesof illumination used for capturing the number of review images may beselected based on type of wafer defect the user of the system 10 wishesto review or based on the material of wafer 12. It is also possible tocapture multiple review images using various combinations and variousintensity levels of brightfield and darkfield illumination set by theuser.

In a step 822, the number of review images captured at each of thedefect image capture positions as calculated in the step 420 areconsolidated and saved. The consolidated and saved review imagescaptured at each of the defect image capture positions are then uploadedto the external storage or server for offline review in a step 824.

In a step 826, the wafer 12 (i.e. the current wafer 12 on the wafertable 16) is unloaded and a second wafer is loaded from the wafer stack20 onto the wafer table 16 by the robotic wafer handler 18. In a step828, each of the steps 402 to 422 is repeated for the second wafer.Consolidated and saved first images and second images of defectsdetected on the second wafer are uploaded to the external storage orserver.

In the second mode 800 b of the review process 800, the steps 820 to 828can be repeated any number of times, depending on the number of wafersof the wafer stack 20. Repetition of the steps 820 to 828 results inconsolidation and saving of the captured brightfield review images anddarkfield review images obtained for each wafer 12 of the wafer stack20, and the uploading of the first images and the second images, to theexternal storage or server for an offline review.

The second mode 800 b of the review process 800 allows continuingproduction while user can perform offline review of saved images. Thesecond mode 800 b of the review process 800 allows capturing multipleimages of each defect at various combinations illuminations for offlinereview without affecting machine utilization and improves productivity.

Third Mode 800 c of the Review Process 800

The third mode 800 c of the review process 800 is preferably initializedby a manual input, more specifically an input or command by the user. Ina step 840, the user captures a first review brightfield image and afirst review darkfield image at a first defect image capture position.In a step 842, the user manually inspects or reviews the first reviewbrightfield image and the first review darkfield image captured.Preferably, the first review brightfield image and the first reviewdarkfield image are displayed on a screen or monitor for facilitatingvisual inspection thereof by the user. The user is able to view thedefect at different illumination combination using the brightfield andthe darkfield illuminator.

In a step 844, the user either accepts or rejects or reclassifies thedefect corresponding to the first defect image capture position. Thesteps 840 to 844 are then sequentially repeated for each and everydefect image capture positions as calculated in the step 420.

After the steps 840 to 844 are sequentially repeated for each and everydefect image capture positions, positive defects and theirclassifications are then consolidated and saved in a step 846. Theconsolidated and saved positive defects and their classifications arethen uploaded or transferred to the external storage or server in a step848. In the third mode 800 c of the review process 800, the wafer 12(i.e. the current wafer 12 on the wafer table 16) is only unloaded afterthe completion of the step 846. Accordingly, it will be appreciated by aperson skilled in the art that the third mode 800 c of the reviewprocess requires continuous user presence or input for effecting thevisual inspection or review of each wafer.

In a step 848 of the review process 800, the wafer 12 (i.e. the currentwafer 12 on the wafer table 16) is unloaded and the robotic waferhandler 18 then loads a second wafer onto the wafer table 16 from thewafer stack 20. The steps 840 to 848 are repeated any number of timesdepending on the number of wafers to be inspected (or number of wafersin the wafer stack 20).

It will be understood by a person skilled in the art with the disclosureprovided by the description above that the first mode 800 a and thesecond mode 800 b of the review process 800 effects a relativelyindiscriminate consolidation, storage and uploading of captured imagesto the external storage or server. The first mode 800 a and the secondmode 800 b represent automated review processes. The user is able toaccess the external storage or server for offline review of the capturedimages as and when required. The first mode 800 a and the second mode800 b enable continuous review of each of the wafers 12 of the waferstack 20, or the continuous image capture, consolidation, upload andstorage.

It will be appreciated by a person skilled in the art that while onlythree review modes, namely the first mode 800 a, the second mode 800 band the third mode 800 c are described in the present description, aperson skilled in the art may employ alternative review processes ordifferent permutations or combinations of the steps of each of the threereview modes 800 a, 800 b and 800 c. In addition, it will be appreciatedby a person skilled in the art that each of the three review modes 8000a, 800 b and 800 c may be modified or altered as required using methodsknown in the art without departing from the scope of the presentinvention.

After the performance of the review process 800, verified defects, andthe locations and classifications thereof, are consolidated and saved ina step 426. The verified defects, and the locations and classificationsthereof; are consolidated and saved either in the database or in anexternal database or memory space. The wafer map is also updated in thestep 426.

As previously described, each of the captured brightfield images, DHAimages and DLA images is compared with a corresponding golden referenceor reference image for identifying or detecting defects on the wafer 12.The exemplary reference image creation process 900 provided by thepresent invention (as shown in FIG. 18) facilitates creation orderivation of such reference images. It will be understood by a personskilled in the art that the reference image creation process 900 canalso be referred to as a training process.

As previously described, each of the 2D brightfield images, 2D DHAimages, 2D DLA images captured during the 2D wafer scanning process 500are preferably matched with their corresponding reference images createdby the reference image creation process 900.

An exemplary comparison process is already described with the 2D imageprocessing process 600. However, for increased clarity, a summary ofmatching between working images and reference images is provided below.Firstly, subpixel alignment of the selected working image is performedusing known references including, but not limited to, templates, trace,bumps, pads and other unique patterns. Secondly, the referenceintensities of the working images of the wafer 12 captured atpredetermined image capture positions are calculated. In other words, aplurality of reference intensities for each of the plurality of pixelsof each working image of the wafer 12 is determined. A plurality ofstatistical parameters for the plurality of reference intensities ofeach of the plurality of pixels of each working image of the wafer 12 isthen calculated. An appropriate reference image for comparing ormatching with the working image (or reference images for comparing withthe working images) is then selected. The appropriate reference image ispreferably selected from the multiple reference images created by thereference image creation process 900. Reference images for comparisonwith working images are selected based on the calculated plurality ofstatistical parameters.

The CPU is preferably programmed for enabling selection and extractionof the appropriate reference image to which the working image will becompared or matched. More specifically, the CPU is preferably programmedfor selecting reference images for comparison with working images basedon the calculated plurality of statistical parameters. Preferably, thecalculation, and storage, of the normalized average or geometric mean,standard deviation, maximum and minimum intensity (collectively known asstatistical parameters) of each pixel of the reference images by thereference image creation process 900 enhances speed and accuracy ofselecting or extracting the appropriate reference image to which theworking image will be compared.

Corresponding quantitative data for each pixel of the working image isthen calculated. The quantitative data is for example normalized averageor geometric mean, standard deviation, maximum and minimum intensitiesof each pixel of the working image. The quantitative data values foreach pixel of the working image is then referenced or checked againstcorresponding data values of each pixel of the selected reference image.

Comparison of quantitative data values between pixels of the workingimage and pixels of the reference image enables identification ordetection of defects. Preferably, predetermined threshold values are setby the user. Difference between the quantitative data values of pixelsof the working image and pixels of the reference image are matchedagainst the predetermined threshold values with one of multiplicative,additive and a constant value. If the difference between thequantitative data values of pixels of the working image and pixels ofthe reference image is greater than the predetermined threshold values,a defect (or defects) is flagged.

The predetermined threshold value can be varied as required. Preferably,the predetermined threshold value is varied for adjusting stringency ofthe method 400. In addition, the predetermined threshold value ispreferably varied as required depending on type of defect to bedetected, material of wafer 12 presented for inspection, or illuminationconditions. Furthermore, the predetermined threshold value may bealtered depending on a customer's or more generally the semiconductorindustry's, requirements.

An exemplary system 10 and an exemplary method 400 for inspectingsemiconductor wafers are described above. A person skilled in the artprovided with the description above will understand that themodifications to the system 10 and the method 400 may be done withoutdeparting from the scope of the present invention. For example, sequenceof steps of the method 400, and the sequence of steps of processes 500,600, 700, 750, 800 and 900, may be modified without departing from thescope of the present invention.

It is an objective of the system 10 and method 400 of the presentinvention to enable accurate and cost-effective inspection ofsemiconductor components, for example wafers. The ability for anautomated inspection of wafers by the system 10 and the method 400 whilethe wafer is in motion enhances efficiency of the inspection of wafers.This is because time is not wasted for decelerating and stopping ofindividual wafers at an inspection position for image capture thereof,and for subsequent acceleration and transport of the wafer from theinspection position after the images have been captured, as with severalexisting semiconductor wafer inspection systems. Known image offsetsbetween multiple image captures facilitate processing of the capturedimages to thereby detect defects that may be present therein. The offsetrelative to the particular set of images for the same wafer enables thesoftware to accurately determine the co-ordinates of the defect in thewafer and, subsequently, the position of the wafer in the entire frame.The offset is preferably determined by reading the encoder values inboth the X and Y displacement motors and is used to calculate theco-ordinates of a defect or defects. In addition, the use of two imagesat every inspect locations combines advantages of two different imagingtechniques for facilitating more accurate wafer inspection.

It will also be understood by a person skilled in the art that thetime-synchronization of image captures can be altered as required. Morespecifically, the time-synchronization may be adjusted for enhancing theability of the programmable controller to compensate for image offsetbetween the captured images. The system 10 and method 400 of the presentinvention facilitates accurate synchronization between supply ofillumination and exposures of corresponding image capture devices forcapturing of images to minimize degradation of inspection quality.

Illuminations used with the system 10 can be in the full visiblespectrum of light for capture of enhanced quality images. Intensities ofillumination and their combinations supplied for capture of images bythe system 10 can be easily selected and varied as required depending onfactors including, but not limited to, type of defects to be detected,material of the wafer and stringency of wafer inspection. The system 10and method 400 provided by the present invention also enablesmeasurement of height of 3D elements on the wafer, and analysis of 3Dprofile images while the wafer 12 is moving.

The system 10 of the present invention has an optical setup, which doesnot require frequent spatial reconfigurations to cater to changes inwafer structure or characteristics. In addition, the use of tube lenses36, 38, 60, 72 with the system 10 enables ease of reconfiguration anddesign of the system 10. The use of tube lenses 36, 38, 60, 72 enhancesease of introduction of optical components and accessories into thesystem 10, more specifically between objective lenses 40 or objectivelens mechanism and the tube lenses 36, 38, 60, 72.

The system 10 of the present invention comprises vibration isolators 24(collectively known as a stabilizer mechanism) for buffering unwantedvibrations to the system 10. The vibration isolators 24 helps to enhancequality of images captured by the first image capture device 32, thesecond image capture device 34, the 3D profile camera 56 and the reviewimage capture device 62, and thus the accuracy of defect detection. Inaddition, the XY table of the system 10 enables accurate displacementand alignment of the wafer 12 relative the inspection position.

As described in the background, existing reference image derivation orcreation processes requires manual selection of “good” wafers, resultingin relative inaccuracies and inconsistencies of derived referenceimages. Accordingly, quality of wafer inspection is adversely affected.The system 10 and method 400 of the present invention achieves enhancedquality of inspection by creating reference images without manualselection (i.e. subjective selection) of “good” wafers. The referenceimage creation process 900 allows for application of differentthresholds of intensities across different locations of the wafer, thusaccommodating non-linear illumination variations across the wafer 12.The method 400 therefore facilitates reduction in false or unwanteddetection of defects and ultimately an enhanced quality of waferinspection.

The present invention enables automated defect detection using ananalytical model or software that compares reference images withcaptured images of unknown quality wafers. The present invention alsoenables automated defect detection, preferably by performing digitalanalysis on digitalized images (i.e. working images and referenceimages).

The present invention enables automated review mode withoutsignificantly affecting production or inspection of wafers. In addition,the present invention helps improve system or machine utilization.Existing equipment or inspection systems typically offer only manualreview mode, which requires the operator to decide or determine each andevery defect manually and while considering multiple factors andparameters, for example different illumination intensities.

In the foregoing manner, an exemplary system and an exemplary method forinspecting semiconductor wafers and components provided by embodimentsof the present invention are described. The exemplary system and methodaddresses at least one of the issues or problems faced by existingsemiconductor inspection systems and methods as mentioned in thebackground. It will however be understood by a person skilled in the artthat the present invention is not limited to specific forms,arrangements or structures of the embodiments described above. It willbe apparent to a person skilled in the art in view of this disclosurethat numerous changes and/or modifications can be made without departingfrom the scope and spirit of the invention.

The invention claimed is:
 1. A method for capturing images of a waferwhile the wafer is displaced along a scan motion path that positions thewafer relative to an inspection position beneath an objective lens suchthat each of a plurality of predetermined areas of the wafer issequentially positioned within an inspection area, the methodcomprising: positioning a first area within the plurality ofpredetermined areas of the wafer within the inspection area as the waferis displaced along the scan motion path; performing a wafer scanningprocess while the first area of the wafer remains within the inspectionarea during displacement of the wafer along the scan motion path, thewafer scanning process comprising: during a first time interval: (i)initiating exposure of a first image capture device, (ii) capturing afirst image of the first area of a wafer at a first image captureposition within the inspection area using the first image capturedevice, under a first contrast illumination supplied by way of a firstflash or strobe, (iii) generating first image data using the first imagecapture device, and (iv) terminating exposure of the first image capturedevice after capture of the first image thereby terminating the firsttime interval; during a second time interval that begins immediatelyupon termination of the first time interval: (v) initiating exposure ofa second image capture device, (vi) capturing a second image of thefirst area of the wafer at a second image capture position within theinspection area using the second image capture device, under a secondcontrast illumination supplied by way of a second flash or strobe, thesecond image having an image offset from the first image due to thewafer being spatially displaced by a predetermined distance between thecapture of the first image and the capture of the second image, (vii)generating second image data using the second image capture device, and(viii) terminating exposure of the second image capture device aftercapture of the second image thereby terminating the second timeinterval; and correlating the first image and the second image bydetermining the image offset between the first image and the secondimage, positioning a second area within the plurality of predeterminedareas of the wafer within the inspection area as the wafer is displacedalong the scan motion path; repeating the wafer scanning process withrespect to the second area of the wafer while the second area of thewafer remains within the inspection area during displacement of thewafer along the scan motion path, to thereby capture: a first image ofthe second area of the wafer at the first image capture position withinthe inspection area using the first image capture device under the firstcontrast illumination; and a second image of the second area of thewafer at the second image capture position within the inspection areausing the second image capture device under the second contrastillumination, wherein a time difference between (a) positioning thefirst area of the wafer within the inspection area for capturing thefirst image of the first area of the wafer, and (b) positioning thesecond area of the wafer within the inspection area for capturing thefirst image of the second area of the wafer equals (i) a time requiredfor exposing the first image capture device and capturing the firstimage of the first area of the wafer, plus (ii) a time required forconverting the first image of the first area of the wafer into firstimage signals and transferring the first image signals to a database ofstorage memory, wherein the first and second image capture devices havean identical field of view within the inspection area for capturing thefirst and second images thereby, respectively.
 2. The method as in claim1, wherein each of the first contrast illumination and the secondcontrast illumination is broadband illumination.
 3. The method as inclaim 1, wherein the first contrast illumination is brightfieldillumination and the first image is a brightfield image, and wherein thesecond contrast illumination is darkfield illumination and the secondimage is a darkfield image.
 4. The method as in claim 1, wherein each ofthe first contrast illumination and the second contrast illumination isselectable from and directable to the inspection position as any one ofbrightfield illumination, darkfield illumination, and a combination ofbrightfield illumination and darkfield illumination.
 5. The method as inclaim 4, wherein each of the first contrast illumination and the secondcontrast illumination is selectable from and directable to theinspection position as any one of brightfield illumination, darkfieldhigh angle illumination, darkfield low angle illumination, and acombination thereof.
 6. The method as in claim 2, wherein each of thefirst contrast illumination and the second contrast illumination isbroadband illumination of a substantially equal wavelength spectrum. 7.The method as in claim 3, wherein the brightfield illumination isemitted by a flash lamp.
 8. The method as in claim 3, wherein thebrightfield illumination is white light illumination.
 9. The method asin claim 1, further comprising comparing at least one defect site of thefirst image with at least one defect site of the second image to producean identification of a defect after correlating the first image and thesecond image.
 10. The method as in claim 1, wherein determining theimage offset between the first image and the second image comprises:retrieving XY encoder values representing first and second waferpositions corresponding to the capture of the first image and the secondimage, respectively; and calculating a coarse offset between the firstimage and the second image based upon the retrieved XY encoder values.11. The method as in claim 10, wherein determining the image offsetbetween the first image and the second image further comprisescalculating a fine offset between the first image and the second imageby way of sub pixel alignment of the first image and the second image.12. A system for capturing images of a wafer while the wafer isdisplaced along a scan motion path that positions the wafer relative toan inspection position beneath an objective lens such that each of aplurality of predetermined areas of the wafer is sequentially positionedwithin an inspection area, the system comprising: a first image capturedevice comprising a first image sensor for (a) capturing a first imageof a first area of the wafer under a flash or strobe of first contrastillumination during a first exposure interval while the first area ofthe wafer is positioned within the inspection area during displacementof the wafer along the scan motion path, (b) generating first image datacorresponding to the first image, and (c) capturing a first image of asecond area of the wafer at the first image capture position within theinspection area under the first contrast illumination; a second imagecapture device comprising a second image sensor for (d) sequentiallycapturing a second image of the first area of the wafer under a flash orstrobe of second contrast illumination during a second exposure intervalthat is separate from the first exposure interval and which is initiatedimmediately upon termination of the first exposure interval while thefirst area of the wafer is positioned within the inspection area duringdisplacement of the wafer along the scan motion path, (e) generatingsecond image data corresponding to the second image, the second imagehaving an image offset from the first image due to the wafer beingspatially displaced by a predetermined distance between the capture ofthe first image and the capture of the second image, and (f) capturing asecond image of the second area of the wafer at the second image captureposition within the inspection area under the second contrastillumination; and a processor coupled to the first and second imagecapture devices and configured for correlating the first and secondimage with the spatial displacement of the wafer by determining theimage offset between the first image and the second image, comparing adefect site detected in the first image with another defect sitedetected in the second image, and producing an identification of adefect therefrom, wherein a time difference between (a) positioning thefirst area of the wafer within the inspection area for capturing thefirst image of the first area of the wafer, and (b) positioning thesecond area of the wafer within the inspection area for capturing thefirst image of the second area of the wafer equals (i) a time requiredfor exposing the first image capture device and capturing the firstimage of the first area of the wafer, plus (ii) a time required forconverting the first image of the first area of the wafer into firstimage signals and transferring the first image signals to a database ofstorage memory, wherein the first and second image capture devices havean identical field of view within the inspection area for capturing thefirst and second images thereby, respectively.
 13. The system as inclaim 12, wherein the processor is further configured for sorting thewafer based on the identification.
 14. The system as in claim 13,wherein the identification is a positive identification of a defect whenthe defect site detected in the first image corresponds with the defectsite detected in the second image, and wherein the identification is anegative identification of a defect otherwise.
 15. The system as inclaim 14, wherein the processor is further configured for sorting thewafer into a first output node and a second output node, and wherein thewafer is sorted to the first output node when the identification is apositive identification and the wafer is sorted to the second node whenthe identification is a negative identification.
 16. The system as inclaim 12, wherein each of the first contrast illumination and the secondcontrast illumination is broadband illumination.
 17. The system as inclaim 16, further comprising an illumination configurator for selectablydirecting to the inspection position a supply of brightfieldillumination, darkfield illumination, and a combination thereof as eachof the first contrast illumination and the second contrast illumination.18. The system as in claim 17, wherein the illumination configuratorselectably directing to the inspection position a supply of brightfieldillumination, darkfield high angle illumination, darkfield low angleillumination, and any combination thereof as each of the first contrastillumination and the second contrast illumination.
 19. The system as inclaim 12, wherein the processor is configured for determining the imageoffset to correlate the first image and the second image with thespatial displacement of the wafer by: retrieving XY encoder valuescorresponding to the first image capture position and the second imagecapture position; and calculating a coarse offset between the firstimage and the second image based upon the retrieved XY encoder values.20. The system as in claim 19, wherein the processor is furtherconfigured for determining the image offset to correlate the first imageand the second image with the spatial displacement of the wafer bycalculating a fine offset between the first image and the second imageby way of sub pixel alignment of the first image and the second image.21. The method as in claim 1, wherein each predetermined area of theplurality of predetermined areas of the wafer comprises at least one dieor a portion thereof.
 22. The method as in claim 6, wherein thebrightfield illumination is emitted by a flash lamp.
 23. The method asin claim 9, wherein the identification of the defect is a positiveidentification if one of the defect sites of the first image correspondswith one of the defect sites of the second image, and wherein theidentification of the defect is a negative identification if none of thedefect sites of the first and second images correspond.
 24. The systemas in claim 12, wherein each predetermined area of the plurality ofpredetermined areas of the wafer comprises at least one die or a portionthereof.
 25. The method as in claim 1, wherein positioning the firstarea of the wafer within the inspection area as the wafer is displacedalong the scan motion path comprises positioning the first area of thewafer under the objective lens such that the objective lens opticallymagnifies the first area of the wafer, and wherein after reflection fromthe surface of the wafer the first contrast illumination and the secondcontrast illumination pass through the objective lens prior torespective capture of the first image by the first image capture deviceunder the first contrast illumination and capture of the second image bythe second image capture device under the second contrast illumination.